Do not substitute #[A-Z] in graphite target when invalid ref (#5581)

* Do not substitute #[A-Z] in graphite target when invalid ref

In the case where #[A-Z] does not reference another query, such as #J when there
are only 2 queries, do not perform the substitution. This prevents the situation
target name without a query labelled J (10th query).

* Do not substitute #[A-Z] in graphite target when invalid ref

In the case where #[A-Z] does not reference another query, such as #J when there
are only 2 queries, do not perform the substitution. This prevents the situation
target name without a query labelled J (10th query).

public/app/plugins/datasource/graphite/datasource.ts

@@ -228,7 +228,7 @@ export function GraphiteDatasource(instanceSettings, $q, backendSrv, templateSrv
     }
 
     function nestedSeriesRegexReplacer(match, g1) {
-      return targets[g1];
+      return targets[g1] || match;
     }
 
     for (i = 0; i < options.targets.length; i++) {