fix empty sparse vector in test

Author: jimczi

server/src/internalClusterTest/java/org/elasticsearch/search/query/ExistsIT.java

@@ -88,9 +88,9 @@ public void testExists() throws Exception {
             // object fields
             singletonMap("bar", barObject),
             singletonMap("bar", singletonMap("baz", 42)),
-            // sparse_vector field empty
-            singletonMap("vec", emptyMap()),
-            // sparse_vector field non-empty
+            // sparse_vector field
+            singletonMap("vec", singletonMap("6", 100)),
+            // sparse_vector field
             singletonMap("vec", singletonMap("1", 100)),
             // empty doc
             emptyMap() };