Submit requirement: Safety net

When front-end feature flag for new submit requirements is enabled,
and back-end doesnt' return any submit requirements. Show old
submit requirement UI with warning.

Screenshot: https://imgur.com/a/LWnINfl
Change-Id: I04285b23c2b5fc8a92eb14075867322034ce1cae
diff --git a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.ts b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.ts
index ba62ec3..8b46cd8 100644
--- a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.ts
@@ -752,6 +752,16 @@
           })
       );
   }
+
+  _showNewSubmitRequirements(change?: ParsedChangeInfo) {
+    if (!this._isSubmitRequirementsUiEnabled) return false;
+    return (change?.submit_requirements ?? []).length > 0;
+  }
+
+  _showNewSubmitRequirementWarning(change?: ParsedChangeInfo) {
+    if (!this._isSubmitRequirementsUiEnabled) return false;
+    return (change?.submit_requirements ?? []).length === 0;
+  }
 }
 
 declare global {
diff --git a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts
index 26d1277..97101f6 100644
--- a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_html.ts
@@ -113,6 +113,10 @@
       --iron-icon-height: 18px;
       --iron-icon-width: 18px;
     }
+    .submit-requirement-error {
+      color: var(--deemphasized-text-color);
+      padding-left: var(--metadata-horizontal-padding);
+    }
   </style>
   <gr-external-style id="externalStyle" name="change-metadata">
     <div class="metadata-header">
@@ -480,20 +484,25 @@
       </span>
     </section>
     <div class="separatedSection">
-      <template is="dom-if" if="[[_isSubmitRequirementsUiEnabled]]">
+      <template is="dom-if" if="[[_showNewSubmitRequirements(change)]]">
         <gr-submit-requirements
           change="[[change]]"
           account="[[account]]"
           mutable="[[_mutable]]"
         ></gr-submit-requirements>
       </template>
-      <template is="dom-if" if="[[!_isSubmitRequirementsUiEnabled]]">
+      <template is="dom-if" if="[[!_showNewSubmitRequirements(change)]]">
         <gr-change-requirements
           change="{{change}}"
           account="[[account]]"
           mutable="[[_mutable]]"
         ></gr-change-requirements>
       </template>
+      <template is="dom-if" if="[[_showNewSubmitRequirementWarning(change)]]">
+        <div class="submit-requirement-error">
+          New Submit Requirements don't work on this change.
+        </div>
+      </template>
     </div>
     <section
       id="webLinks"