Handle range query edge case (elastic#63397)

Currently when searching with an empty string as lower bound for a range query
on text-based fields we return all documents when 'gte' is used (including the
lower bound) but no documents when 'gt' is used. This might seem
counterintuitive since every value should be greate than the empty string. 
The bug has been fixed in Lucene and this PR adds a test for assuring we observe
the fixed behaviour on searches now.

Closes elastic#63386

server/src/internalClusterTest/java/org/elasticsearch/search/simple/SimpleSearchIT.java

@@ -225,6 +225,51 @@ public void testSimpleDateRange() throws Exception {
         assertHitCount(searchResponse, 2L);
     }
 
+    public void testRangeQueryKeyword() throws Exception {
+        createIndex("test");
+
+        client().admin().indices().preparePutMapping("test").setSource("field", "type=keyword").get();
+
+        client().prepareIndex("test").setId("0").setSource("field", "").get();
+        client().prepareIndex("test").setId("1").setSource("field", "A").get();
+        client().prepareIndex("test").setId("2").setSource("field", "B").get();
+        client().prepareIndex("test").setId("3").setSource("field", "C").get();
+        ensureGreen();
+        refresh();
+
+        SearchResponse searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gte("A").lte("B")).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 2L);
+
+        searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gt("A").lte("B")).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 1L);
+
+        searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gte("A").lt("B")).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 1L);
+
+        searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gte(null).lt("C")).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 3L);
+
+        searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gte("B").lt(null)).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 2L);
+
+        searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gt(null).lt(null)).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 4L);
+
+        searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gte("").lt(null)).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 4L);
+
+        searchResponse = client().prepareSearch("test").setQuery(QueryBuilders.rangeQuery("field").gt("").lt(null)).get();
+        assertNoFailures(searchResponse);
+        assertHitCount(searchResponse, 3L);
+    }
+
     public void testSimpleTerminateAfterCount() throws Exception {
         prepareCreate("test").setSettings(Settings.builder().put(SETTING_NUMBER_OF_SHARDS, 1).put(SETTING_NUMBER_OF_REPLICAS, 0)).get();
         ensureGreen();