add graphite function isNonNull

src/app/services/graphite/gfunc.js

@@ -69,6 +69,11 @@ function (_) {
     category: categories.Combine,
   });
 
+  addFuncDef({
+    name: 'isNonNull',
+    category: categories.Combine,
+  });
+
   addFuncDef({
     name: 'rangeOfSeries',
     category: categories.Combine