fix(webapp): guard Flux viewer against js-yaml 5.x empty-input throw

js-yaml 5.x (merged on 2.x via #8863) changed load('') to throw a
YAMLException ('expected a document, but the input is empty') instead of
returning undefined (see js-yaml migrate_v4_to_v5). The Flux Topology
Viewer calls parseAndRender() on page load while the textarea holds only
a comment (# YAML Definition), so jsyaml.load() now throws before the
existing if(doc==null) guard, breaking the viewer at runtime.

This latent bug shipped untested on 2.x because cypress-tests.yml was
scoped to master only, so 2.x PRs never ran the e2e suite.

Wrap the load in try/catch and treat empty/comment-only or malformed
input as 'no document', and broaden cypress-tests.yml to run on 2.x.

Author: rzo1

.github/workflows/cypress-tests.yml

@@ -17,11 +17,11 @@ name: Cypress E2E Tests
 
 on:
   push:
-    branches: [ "master" ]
+    branches: [ "master", "2.x" ]
     paths:
       - 'storm-webapp/**'
   pull_request:
-    branches: [ "master" ]
+    branches: [ "master", "2.x" ]
     paths:
       - 'storm-webapp/**'
   workflow_dispatch:

storm-webapp/src/main/java/org/apache/storm/daemon/ui/WEB-INF/flux.html

@@ -70,7 +70,14 @@
 
       function parseAndRender() {
         var input = document.getElementById('taInput').value;
-        var doc = jsyaml.load(input);
+        var doc;
+        try {
+          doc = jsyaml.load(input);
+        } catch (e) {
+          // js-yaml >=5 throws on empty/comment-only input instead of
+          // returning undefined (see migrate_v4_to_v5); treat as no document.
+          return;
+        }
 	if(doc==null){
 	   return;
 	}