fix(FormGroup): set size default to null

This prevents passing a `size` prop when not specified, especially when having a Checkbox, Radio, etc. underneath that don't support this prop.

src/runtime/components/forms/FormGroup.ts

@@ -19,7 +19,7 @@ export default defineComponent({
     },
     size: {
       type: String,
-      default: () => appConfig.ui.input.default.size,
+      default: null,
       validator (value: string) {
         return Object.keys(appConfig.ui.formGroup.size).includes(value)
       }