fix: Type fixes for ValueChanged functions

Author: danvick

lib/src/fields/form_builder_date_range_picker.dart

@@ -25,7 +25,7 @@ class FormBuilderDateRangePicker extends FormBuilderField<DateTimeRange> {
   final MaxLengthEnforcement? maxLengthEnforcement;
   final int? maxLength;
   final VoidCallback? onEditingComplete;
-  final ValueChanged<List<DateTime>>? onFieldSubmitted;
+  final ValueChanged<DateTimeRange?>? onFieldSubmitted;
   final List<TextInputFormatter>? inputFormatters;
   final double cursorWidth;
   final Radius? cursorRadius;

lib/src/fields/form_builder_date_time_picker.dart

@@ -69,7 +69,7 @@ class FormBuilderDateTimePicker extends FormBuilderField<DateTime> {
 
   /// Called when an enclosing form is submitted. The value passed will be
   /// `null` if [format] fails to parse the text.
-  final ValueChanged<DateTime>? onFieldSubmitted;
+  final ValueChanged<DateTime?>? onFieldSubmitted;
   final TextEditingController? controller;
   final TextInputType keyboardType;
   final TextStyle? style;

lib/src/fields/form_builder_text_field.dart

@@ -155,7 +155,7 @@ class FormBuilderTextField extends FormBuilderField<String> {
   ///  * [EditableText.onSubmitted] for an example of how to handle moving to
   ///    the next/previous field when using [TextInputAction.next] and
   ///    [TextInputAction.previous] for [textInputAction].
-  final ValueChanged<String>? onSubmitted;
+  final ValueChanged<String?>? onSubmitted;
 
   /// {@macro flutter.widgets.editableText.inputFormatters}
   final List<TextInputFormatter>? inputFormatters;