fix: Just use shouldEncryptElement for the envelope "validation"

packages/moxxmpp/lib/src/xeps/xep_0384/xep_0384.dart

@@ -459,9 +459,15 @@ abstract class BaseOmemoManager extends XmppManagerBase {
         );
       }
 
-      children.addAll(
-        envelope.firstTag('content')!.children,
-      );
+      final envelopeChildren = envelope.firstTag('content')?.children;
+      if (envelopeChildren != null) {
+        children.addAll(
+          // Do not add forbidden elements from the envelope 
+          envelopeChildren.where(shouldEncryptElement),
+        );
+      } else {
+        logger.warning('Invalid envelope element: No <content /> element');
+      }
 
       if (!checkAffixElements(envelope, stanza.from!, ourJid)) {
         other['encryption_error'] = InvalidAffixElementsException();