Enhance NFS mount option check for empty response

```
ERROR: test suite for <class 'tests.component.maint.test_primary_storage_nfsmountopts_kvm.TestNFSMountOptsKVM'>
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/nose/suite.py", line 210, in run
    self.setUp()
  File "/usr/local/lib/python3.9/site-packages/nose/suite.py", line 293, in setUp
    self.setupContext(ancestor)
  File "/usr/local/lib/python3.9/site-packages/nose/suite.py", line 316, in setupContext
    try_run(context, names)
  File "/usr/local/lib/python3.9/site-packages/nose/util.py", line 471, in try_run
    return func()
  File "/marvin/tests/component/maint/test_primary_storage_nfsmountopts_kvm.py", line 46, in setUpClass
    cls.version = cls.getNFSMountOptionForPool(cls, "vers", cls.storage_pool.id)
  File "/marvin/tests/component/maint/test_primary_storage_nfsmountopts_kvm.py", line 115, in getNFSMountOptionForPool
    stat = nfsstat[0]
IndexError: list index out of range
```

because 

```
shClient: DEBUG: {Cmd: nfsstat -m | sed -n '/e08df33d-d187-3c1c-88f4-10c69669f41a/{ n; p }' via Host: 10.0.32.210} {returns: []}
```

Author: DaanHoogland

test/integration/component/maint/test_primary_storage_nfsmountopts_kvm.py

@@ -110,7 +110,7 @@ def getUnusedNFSVersions(self, filter):
     def getNFSMountOptionForPool(self, option, poolId):
         nfsstat_cmd = "nfsstat -m | sed -n '/%s/{ n; p }'" % poolId
         nfsstat = self.sshClient.execute(nfsstat_cmd)
-        if (nfsstat == None):
+        if nfsstat == None or len(nfsstat) == 0:
             return None
         stat = nfsstat[0]
         vers = stat[stat.find(option):].split("=")[1].split(",")[0]