Guard against groups with no UUID

prevent issue whereby groups with empty UUID would be loaded, giving the
following error:

```
com.google.common.util.concurrent.UncheckedExecutionException:
java.lang.IllegalArgumentException: UUIDs must consist of at least two
characters at
com.github.benmanes.caffeine.guava.CaffeinatedGuavaLoadingCache.get(CaffeinatedGuavaLoadingCache.java:65)

```

Check whether the group has a valid UUID and skip otherwise

Change-Id: I93d69bc07a08baa73ec4345d3af77d3e4f8d87f0
diff --git a/admin/warm-cache-1.0.groovy b/admin/warm-cache-1.0.groovy
index 46c34f1..88a8148 100644
--- a/admin/warm-cache-1.0.groovy
+++ b/admin/warm-cache-1.0.groovy
@@ -89,6 +89,9 @@
     def groupsLoaded = 0
 
     for (groupUuid in allGroupsUuids) {
+      if(groupUuid.get().length() == 0) {
+        continue
+      }
       groupIncludeCache.parentGroupsOf(groupUuid)
       def group = groupCache.get(groupUuid)