Add std.isEmpty for string (#1059)

* Add std.isEmpty for string

* added doc for `availableSince: 'upcoming'`

Author: Tejesh-Raut

doc/_stdlib_gen/stdlib-content.jsonnet

@@ -425,6 +425,14 @@ local html = import 'html.libsonnet';
             },
           ],
         },
+        {
+          name: 'isEmpty',
+          params: ['str'],
+          availableSince: 'upcoming',
+          description: |||
+            Returns true if the the given string is of zero length.
+          |||,
+        },
         {
           name: 'asciiUpper',
           params: ['str'],

stdlib/std.jsonnet

@@ -1692,4 +1692,6 @@ limitations under the License.
   xor(x, y):: x!=y,
 
   xnor(x, y):: x==y,
+
+  isEmpty(str):: std.length(str) == 0,
 }

test_suite/stdlib.jsonnet

@@ -1550,4 +1550,7 @@ std.assertEqual(std.xor(true, true), false) &&
 std.assertEqual(std.xnor(true, false), false) &&
 std.assertEqual(std.xnor(true, true), true) &&
 
+std.assertEqual(std.isEmpty(""), true) &&
+std.assertEqual(std.isEmpty("non-empty string"), false) &&
+
 true