Search: Empty facets element causes search failures, closes #938.

modules/elasticsearch/src/main/java/org/elasticsearch/search/facet/FacetPhase.java

@@ -59,7 +59,7 @@ public class FacetPhase implements SearchPhase {
     }
 
     @Override public void execute(SearchContext context) throws ElasticSearchException {
-        if (context.facets() == null) {
+        if (context.facets() == null || context.facets().facetCollectors() == null) {
             return;
         }
         if (context.queryResult().facets() != null) {

modules/elasticsearch/src/main/java/org/elasticsearch/search/internal/ContextIndexSearcher.java

@@ -78,6 +78,9 @@ public class ContextIndexSearcher extends ExtendedIndexSearcher {
     }
 
     public List<Collector> removeCollectors(String scope) {
+        if (scopeCollectors == null) {
+            return null;
+        }
         return scopeCollectors.remove(scope);
     }