Fix reload behaviour; don't override global with empty local value

src/features/campaigns/TemplateDataPage.tsx

@@ -474,7 +474,7 @@ function buildPreviewContext(draft: Record<string, unknown> | null, entry: Recor
     addContextValue(context, key, "local", value);
   }
   for (const [key, value] of Object.entries(entryFields)) {
-    if (canOverrideField(overridePolicy, key)) {
+    if (canOverrideField(overridePolicy, key) && hasPreviewOverrideValue(value)) {
       addContextValue(context, key, "local", value);
     }
   }
@@ -505,6 +505,12 @@ function addContextValue(context: Record<string, string>, key: string, namespace
   context[`${namespace}::${key}`] = text;
 }
 
+function hasPreviewOverrideValue(value: unknown): boolean {
+  if (value === undefined || value === null) return false;
+  if (typeof value === "string") return value.trim() !== "";
+  return true;
+}
+
 function renderPreviewText(text: string, context: Record<string, string>, ignoreEmptyFields: boolean): string {
   if (!text) return "";
   return text