Loki: Return false from isMetricsQuery if query is empty (#47024)

public/app/plugins/datasource/loki/datasource.test.ts

@@ -13,7 +13,7 @@ import {
   toUtc,
 } from '@grafana/data';
 import { BackendSrvRequest, FetchResponse } from '@grafana/runtime';
-import { LokiDatasource, RangeQueryOptions } from './datasource';
+import { isMetricsQuery, LokiDatasource, RangeQueryOptions } from './datasource';
 import { LokiQuery, LokiResponse, LokiResultType } from './types';
 import { getQueryOptions } from 'test/helpers/getQueryOptions';
 import { TemplateSrv } from 'app/features/templating/template_srv';
@@ -1029,6 +1029,23 @@ describe('LokiDatasource', () => {
   });
 });
 
+describe('isMetricsQuery', () => {
+  it('should return true for metrics query', () => {
+    const query = 'rate({label=value}[1m])';
+    expect(isMetricsQuery(query)).toBeTruthy();
+  });
+
+  it('should return false for logs query', () => {
+    const query = '{label=value}';
+    expect(isMetricsQuery(query)).toBeFalsy();
+  });
+
+  it('should not blow up on empty query', () => {
+    const query = '';
+    expect(isMetricsQuery(query)).toBeFalsy();
+  });
+});
+
 function assertAdHocFilters(query: string, expectedResults: string, ds: LokiDatasource) {
   const lokiQuery: LokiQuery = { refId: 'A', expr: query };
   const result = ds.addAdHocFilters(lokiQuery.expr);

public/app/plugins/datasource/loki/datasource.ts

@@ -811,6 +811,9 @@ export function lokiSpecialRegexEscape(value: any) {
  * Sometimes important to know that before we actually do the query.
  */
 export function isMetricsQuery(query: string): boolean {
+  if (!query) {
+    return false;
+  }
   const tokens = Prism.tokenize(query, syntax);
   return tokens.some((t) => {
     // Not sure in which cases it can be string maybe if nothing matched which means it should not be a function