Fix T83296: Unknown actions no longer perform an undo push

str_len_clip is initialized to 0, so when "Unknown Action"
occurs, set str_len_clip appropriately.
Regression in 0688309988

Ref D9759

source/blender/editors/interface/interface_handlers.c

@@ -842,6 +842,7 @@ static void ui_apply_but_undo(uiBut *but)
     /* fallback, else we don't get an undo! */
     if (str == NULL || str[0] == '\0' || str_len_clip == 0) {
       str = "Unknown Action";
+      str_len_clip = strlen(str);
     }
 
     /* Optionally override undo when undo system doesn't support storing properties. */