fix(server): add tip for gremlin api NPE with empty query (#2467)

fix #2426

Author: SunnyBoy-WYH

hugegraph-server/hugegraph-api/src/main/java/org/apache/hugegraph/api/gremlin/GremlinAPI.java

@@ -20,6 +20,7 @@
 import org.apache.hugegraph.api.filter.CompressInterceptor.Compress;
 import org.apache.hugegraph.config.HugeConfig;
 import org.apache.hugegraph.metrics.MetricsUtil;
+import org.apache.hugegraph.util.E;
 
 import com.codahale.metrics.Histogram;
 import com.codahale.metrics.annotation.Timed;
@@ -77,6 +78,7 @@ public Response get(@Context HugeConfig conf,
                         @Context UriInfo uriInfo) {
         String auth = headers.getHeaderString(HttpHeaders.AUTHORIZATION);
         String query = uriInfo.getRequestUri().getRawQuery();
+        E.checkArgumentNotNull(query, "The request query can't be empty");
         MultivaluedMap<String, String> params = uriInfo.getQueryParameters();
         Response response = this.client().doGetRequest(auth, params);
         GREMLIN_INPUT_HISTOGRAM.update(query.length());