Allow empty annotation values

Fixes:
openshift#11175

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

**Before**
```
$ oc annotate pod zookeeper-1 openshift.io/node-selector="" --overwrite
error: invalid annotation format: openshift.io/node-selector=
```

**After**
```
$ oc annotate pod zookeeper-1 openshift.io/node-selector="" --overwrite
pod "zookeper-1" annotated
```

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

Author: juanvallejo