Parser throws NullPointerException when Filter aggregation clause is …

…empty.

Added Junit test that recreates the error and fixed FilterParser to default to using a MatchAllDocsFilter if the requested filter clause is left empty.
Also added fix and test for the Filters (with an "s") aggregation.

Closes elastic#8438

src/main/java/org/elasticsearch/search/aggregations/bucket/filter/FilterParser.java

@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.search.aggregations.bucket.filter;
 
+import org.elasticsearch.common.lucene.search.MatchAllDocsFilter;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.index.query.ParsedFilter;
 import org.elasticsearch.search.aggregations.Aggregator;
@@ -39,7 +40,8 @@ public String type() {
     @Override
     public AggregatorFactory parse(String aggregationName, XContentParser parser, SearchContext context) throws IOException {
         ParsedFilter filter = context.queryParserService().parseInnerFilter(parser);
-        return new FilterAggregator.Factory(aggregationName, filter.filter());
+
+        return new FilterAggregator.Factory(aggregationName, filter == null ? new MatchAllDocsFilter() : filter.filter());
     }
 
 }

src/test/java/org/elasticsearch/search/aggregations/bucket/FilterTests.java

@@ -21,6 +21,8 @@
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.index.IndexRequestBuilder;
 import org.elasticsearch.action.search.SearchResponse;
+import org.elasticsearch.index.query.AndFilterBuilder;
+import org.elasticsearch.index.query.FilterBuilder;
 import org.elasticsearch.search.aggregations.bucket.filter.Filter;
 import org.elasticsearch.search.aggregations.bucket.histogram.Histogram;
 import org.elasticsearch.search.aggregations.metrics.avg.Avg;
@@ -97,6 +99,20 @@ public void simple() throws Exception {
         assertThat(filter.getDocCount(), equalTo((long) numTag1Docs));
     }
 
+    // See NullPointer issue when filters are empty:
+    // https://github.com/elasticsearch/elasticsearch/issues/8438
+    @Test
+    public void emptyFilterDeclarations() throws Exception {
+        FilterBuilder emptyFilter = new AndFilterBuilder();
+        SearchResponse response = client().prepareSearch("idx").addAggregation(filter("tag1").filter(emptyFilter)).execute().actionGet();
+
+        assertSearchResponse(response);
+
+        Filter filter = response.getAggregations().get("tag1");
+        assertThat(filter, notNullValue());
+        assertThat(filter.getDocCount(), equalTo((long) numDocs));
+    }
+
     @Test
     public void withSubAggregation() throws Exception {
         SearchResponse response = client().prepareSearch("idx")