fix: --from-ref for a plugin definition missing a variant (meltan…

…o#8379)

Don't error out when adding from a plugin definition that does not contain variant

src/meltano/cli/utils.py

@@ -305,7 +305,7 @@ def add_plugin(  # noqa: C901
         plugin_attrs = plugin_definition.canonical()
 
         plugin_name = plugin_attrs.pop("name")
-        variant = plugin_attrs.pop("variant")
+        variant = plugin_attrs.pop("variant", variant)
 
     try:
         plugin = add_service.add(