Merge pull request #57050 from juanvallejo/jvallejo/check-empty-label-resource-builder

Automatic merge from submit-queue (batch tested with PRs 56676, 57050, 54881, 56822, 57113). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

check for empty label before assigning

Prevents an empty label from being assigned to the resource builder when calling its `LabelSelectorParam` parameter. Currently, we assign the address of the provided selector string, even if the string is empty - this has the potential of returning erroneous failures since we check to see [if a selector was provided here](https://github.com/kubernetes/kubernetes/blob/master/pkg/kubectl/resource/builder.go#L470), for example, by doing a nil check. Storing the pointer to [an empty string value](https://github.com/kubernetes/kubernetes/blob/master/pkg/kubectl/resource/builder.go#L344) would break this check as the pointer is no longer nil

**Release note**:
```release-note
NONE
```

cc @deads2k @smarterclayton

pkg/kubectl/resource/builder.go

@@ -341,6 +341,10 @@ func (b *Builder) LabelSelectorParam(s string) *Builder {
 // LabelSelector accepts a selector directly and will filter the resulting list by that object.
 // Use LabelSelectorParam instead for user input.
 func (b *Builder) LabelSelector(selector string) *Builder {
+	if len(selector) == 0 {
+		return b
+	}
+
 	b.labelSelector = &selector
 	return b
 }