Fix for issue 1350 with some changes to tests. This fix makes it so that valueTransformer can transform values to null.

Author: BrottarBasse

lib/src/form_builder_field.dart

@@ -88,7 +88,8 @@ class FormBuilderFieldState<F extends FormBuilderField<T>, T>
       (_formBuilderState?.initialValue ??
           const <String, dynamic>{})[widget.name] as T?;
 
-  dynamic get transformedValue => widget.valueTransformer == null ? value : widget.valueTransformer!(value);
+  dynamic get transformedValue =>
+      widget.valueTransformer == null ? value : widget.valueTransformer!(value);
 
   @override
   String? get errorText => super.errorText ?? _customErrorText;