label.c: Set empty text by default

This will avoid crashing the application on rendering due to
uninitialized labels.

src/gui/src/label.c

@@ -34,6 +34,8 @@ void gui_label_init(struct gui_label *const l)
         .common =
         {
             .cb = &cb
-        }
+        },
+
+        .text = ""
     };
 }