fix: dropdown-hint-missing

Author: mvarendorff2

lib/src/fields/form_builder_dropdown.dart

@@ -295,6 +295,7 @@ class FormBuilderDropdown<T> extends FormBuilderFieldDecoration<T> {
            final hasValue = items.map((e) => e.value).contains(field.value);
            return InputDecorator(
              decoration: state.decoration,
+             isEmpty: !hasValue,
              child: DropdownButton<T>(
                menuWidth: menuWidth,
                padding: padding,