empty viewport, prevent duplicate series meta from add, protect broadcast events when no data has changed

platform/viewer/src/components/EmptyViewport.jsx

@@ -0,0 +1,11 @@
+import React from 'react';
+
+function EmptyViewport() {
+  return (
+    <div>
+      <p>Please drag a stack here to view images.</p>
+    </div>
+  );
+}
+
+export default EmptyViewport;