[YANG][Bugfix] Fix incorrect pattern of nexthop-vrf in sonic-static-route.yang (#21642)

#### Why I did it
Fix issue #21352 
According to the Static Route HLD, the nexthop-vrf field should be optional:
<pre>
;Defines IP static route  table
;
;Status: stable

key                 = STATIC_ROUTE|vrf-name|prefix ;
vrf-name            = 1\*15VCHAR ; VRF name
prefix              = IPv4Prefix / IPv6prefix
nexthop             = string; List of gateway addresses;
ifname              = string; List of interfaces
distance            = string; {0..255};List of distances.
                      Its a Metric used to specify preference of next-hop
                      if this distance is not set, default value 0 will be set when this field is not configured for nexthop(s)
nexthop-vrf         = string; list of next-hop VRFs. It should be set only if ifname or nexthop IP  is not
                      in the current VRF . The value is set to VRF name
                      to which the interface or nexthop IP  belongs for route leaks.
blackhole           = string; List of boolean; true if the next-hop route is blackholed.
                      Default value false will be set when this field is not configured for nexthop(s)
</pre>
However, in the sonic-static-route.yang the pattern of nexthop-vrf implies the nexthop-vrf is mandatory.

<pre>
sonic-static-route.yang:
...
...
        leaf nexthop-vrf {
          type string {
            pattern "(((Vrf[a-zA-Z0-9_-]+)|(default)|(mgmt)),)*((Vrf[a-zA-Z0-9_-]+)|(default)|(mgmt))";
          }
          description
            "VRF name of the nexthop. This is for vrf leaking";
...
</pre>


#### How I did it
Correct the pattern of nexthop-vrf.

#### How to verify it
<pre>
admin@sonic:~$ sudo config route add prefix 0.0.0.0/0 nexthop 172.16.0.3
admin@sonic:~$ redis-cli -n 4
127.0.0.1:6379[4]> key *STATIC*
(error) ERR unknown command 'key', with args beginning with: '*STATIC*'
127.0.0.1:6379[4]> keys *STATIC*
1) "STATIC_ROUTE|default|0.0.0.0/0"
127.0.0.1:6379[4]> hgetall "STATIC_ROUTE|default|0.0.0.0/0"
 1) "blackhole"
 2) "false"
 3) "distance"
 4) "0"
 5) "ifname"
 6) ""
 7) "nexthop"
 8) "172.16.0.3"
 9) "nexthop-vrf"
10) ""
127.0.0.1:6379[4]> exit
admin@sonic:~$ sudo config interface breakout Ethernet8 4x100G -yf

Running Breakout Mode : 1x400G
Target Breakout Mode : 4x100G

Ports to be deleted :
 {
    "Ethernet8": "400000"
}
Ports to be added :
 {
    "Ethernet8": "100000",
    "Ethernet10": "100000",
    "Ethernet12": "100000",
    "Ethernet14": "100000"
}
Breakout process got successfully completed.
Please note loaded setting will be lost after system reboot. To preserve setting, run `config save`.
</pre>

Author: puffc

src/sonic-yang-models/tests/yang_model_tests/tests/static_route.json

@@ -52,6 +52,9 @@
      "STATIC_ROUTE_TEST_ECMP_NEXTHOP_VRF_INVALID": {
         "desc": "Configure with invalid vrf for ECMP",
         "eStrKey": "Pattern"
+    },
+    "STATIC_ROUTE_TEST_NEXTHOP_EMPTY_VRF": {
+        "desc": "Configure with null value for VRF"
     }
 
 }

src/sonic-yang-models/tests/yang_model_tests/tests_config/static-route.json

@@ -1021,6 +1021,53 @@
                 }]
             }
         }
+    },
+    "STATIC_ROUTE_TEST_NEXTHOP_EMPTY_VRF": {
+        "sonic-port:sonic-port": {
+            "sonic-port:PORT": {
+                "PORT_LIST": [
+                    {
+                        "admin_status": "up",
+                        "alias": "eth0",
+                        "description": "Ethernet0",
+                        "fec": "rs",
+                        "lanes": "65",
+                        "mtu": 9000,
+                        "name": "Ethernet0",
+                        "speed": 25000
+                    }
+                ]
+            }
+        },
+        "sonic-interface:sonic-interface": {
+            "sonic-interface:INTERFACE": {
+                "INTERFACE_IPPREFIX_LIST": [
+                    {
+                        "family": "IPv4",
+                        "ip-prefix": "1.0.0.1/30",
+                        "name": "Ethernet0",
+                        "scope": "global"
+                    }
+                ],
+                "INTERFACE_LIST": [
+                    {
+                        "name": "Ethernet0"
+                    }
+                ]
+            }
+        },
+        "sonic-static-route:sonic-static-route": {
+            "sonic-static-route:STATIC_ROUTE": {
+                "STATIC_ROUTE_LIST": [{
+                        "prefix": "16.16.16.0/24",
+                        "vrf_name": "default",
+                        "nexthop": "1.0.0.5",
+                        "distance": "1",
+                        "nexthop-vrf": "",
+                        "blackhole": "false"
+                }]
+            }
+        }
     }
 
 }

src/sonic-yang-models/yang-models/sonic-static-route.yang

@@ -111,7 +111,7 @@ module sonic-static-route {
         }
         leaf nexthop-vrf {
           type string {
-            pattern "(((Vrf[a-zA-Z0-9_-]+)|(default)|(mgmt)),)*((Vrf[a-zA-Z0-9_-]+)|(default)|(mgmt))";
+            pattern "((((Vrf[a-zA-Z0-9_-]+)|(default)|(mgmt)),)*((Vrf[a-zA-Z0-9_-]+)|(default)|(mgmt)))?";
           }
           description
             "VRF name of the nexthop. This is for vrf leaking";