Bugfix: do not explode warnings when group_loc is null

src/main/java/api/messages/GoblintMessagesResult.java

@@ -92,7 +92,7 @@ public static class Group implements MultiPiece {
          * @return A collection of AnalysisResult objects.
          */
         public List<AnalysisResult> convert(List<Tag> tags, String severity, boolean explode) {
-            return explode
+            return explode && this.group_loc != null
                     ? convertGroupExplode(tags, severity)
                     : convertGroup(tags, severity);
         }