fix(model-core) apiVersion for core k8s resources

The HasMetadata#getApiVersion method is returning a string starting
with "/" when the group is "". This is not consistent with how owner
references are specified.

This issues is causing:
operator-framework/java-operator-sdk#2723

Author: mnk

Diff for: kubernetes-model-generator/kubernetes-model-core/src/main/java/io/fabric8/kubernetes/api/model/HasMetadata.java

@@ -76,7 +76,11 @@ static String getApiVersion(Class<?> clazz) {
     final String group = getGroup(clazz);
     final String version = getVersion(clazz);
     if (group != null && version != null) {
-      return group + "/" + version;
+      if (group.isEmpty()) {
+        return version;
+      } else {
+        return group + "/" + version;
+      }
     }
     if (group != null || version != null) {
       throw new IllegalArgumentException(

Diff for: kubernetes-model-generator/kubernetes-model-core/src/test/java/io/fabric8/kubernetes/api/model/HasMetadataTest.java

@@ -206,6 +206,11 @@ void addingAndRemovingOwnerReferenceShouldWork() {
     assertFalse(hasMetadata.hasOwnerReferenceFor(owner2));
   }
 
+  @Test
+  void apiVersionOfConfigMapShouldBeCorrect() {
+    assertEquals("v1", HasMetadata.getApiVersion(ConfigMap.class));
+  }
+
   @Test
   void addingSameOwnerReferenceMultipleTimesShouldAddItOnlyOnce() {
     HasMetadata hasMetadata = new Default();