Allow empty annotation values

Annotations with empty values can be used, for example, in diagnostics
logging. This patch removes the client-side check for empty values in
an annotation key-value pair.

**Before**
```
$ kubectl annotate pod zookeeper-1 node-selector="" --overwrite
error: invalid annotation format: node-selector=
```

**After**
```
$ kubectl annotate pod zookeeper-1 node-selector="" --overwrite
pod "zookeper-1" annotated
```

```
$ kubectl get po/zookeeper-1 --template='{{.metadata.annotations}}'
map[... node-selector: test-label:test]
```

pkg/kubectl/cmd/util/helpers.go

@@ -605,7 +605,7 @@ func ParsePairs(pairArgs []string, pairType string, supportRemove bool) (newPair
 	for _, pairArg := range pairArgs {
 		if strings.Index(pairArg, "=") != -1 {
 			parts := strings.SplitN(pairArg, "=", 2)
-			if len(parts) != 2 || len(parts[1]) == 0 {
+			if len(parts) != 2 {
 				if invalidBuf.Len() > 0 {
 					invalidBuf.WriteString(", ")
 				}