[InputDatePickerFormField] adds acceptEmptyDate to InputDatePickerFormField Widget (#124143)

Adds `acceptEmptyDate` property to `InputDatePickerFormField`.

Fixes: #117511

| `acceptEmptyDate` is **false** | `acceptEmptyDate` is **true** |
| --- | ---|
| ![acceptemptydate_false](https://user-images.githubusercontent.com/13456345/229893658-280ecdee-d509-4579-b53c-9d8d485c61b4.gif) | ![acceptemptydate__true](https://user-images.githubusercontent.com/13456345/229895144-115e71bd-e5bb-4653-8db2-9f57dd8262aa.gif) |

Author: piedcipher

packages/flutter/lib/src/material/input_date_picker_form_field.dart

@@ -58,6 +58,7 @@ class InputDatePickerFormField extends StatefulWidget {
     this.fieldLabelText,
     this.keyboardType,
     this.autofocus = false,
+    this.acceptEmptyDate = false,
   }) : initialDate = initialDate != null ? DateUtils.dateOnly(initialDate) : null,
        firstDate = DateUtils.dateOnly(firstDate),
        lastDate = DateUtils.dateOnly(lastDate) {
@@ -130,6 +131,13 @@ class InputDatePickerFormField extends StatefulWidget {
   /// {@macro flutter.widgets.editableText.autofocus}
   final bool autofocus;
 
+  /// Determines if an empty date would show [errorFormatText] or not.
+  ///
+  /// Defaults to false.
+  ///
+  /// If true, [errorFormatText] is not shown when the date input field is empty.
+  final bool acceptEmptyDate;
+
   @override
   State<InputDatePickerFormField> createState() => _InputDatePickerFormFieldState();
 }
@@ -206,6 +214,9 @@ class _InputDatePickerFormFieldState extends State<InputDatePickerFormField> {
   }
 
   String? _validateDate(String? text) {
+    if ((text == null || text.isEmpty) && widget.acceptEmptyDate) {
+      return null;
+    }
     final DateTime? date = _parseDate(text);
     if (date == null) {
       return widget.errorFormatText ?? MaterialLocalizations.of(context).invalidDateFormatLabel;

packages/flutter/test/material/input_date_picker_form_field_test.dart

@@ -49,6 +49,7 @@ void main() {
     Key? formKey,
     ThemeData? theme,
     Iterable<LocalizationsDelegate<dynamic>>? localizationsDelegates,
+    bool acceptEmptyDate = false,
   }) {
     return MaterialApp(
       theme: theme ?? ThemeData.from(colorScheme: const ColorScheme.light()),
@@ -69,6 +70,7 @@ void main() {
             fieldHintText: fieldHintText,
             fieldLabelText: fieldLabelText,
             autofocus: autofocus,
+            acceptEmptyDate: acceptEmptyDate,
           ),
         ),
       ),
@@ -345,5 +347,34 @@ void main() {
         )
       );
     });
+
+    testWidgets('when an empty date is entered and acceptEmptyDate is true, then errorFormatText is not shown', (WidgetTester tester) async {
+      final GlobalKey<FormState> formKey = GlobalKey<FormState>();
+      const String errorFormatText = 'That is not a date.';
+      await tester.pumpWidget(inputDatePickerField(
+        errorFormatText: errorFormatText,
+        formKey: formKey,
+        acceptEmptyDate: true,
+      ));
+      await tester.enterText(find.byType(TextField), '');
+      await tester.pumpAndSettle();
+      formKey.currentState!.validate();
+      await tester.pumpAndSettle();
+      expect(find.text(errorFormatText), findsNothing);
+    });
+
+    testWidgets('when an empty date is entered and acceptEmptyDate is false, then errorFormatText is shown', (WidgetTester tester) async {
+      final GlobalKey<FormState> formKey = GlobalKey<FormState>();
+      const String errorFormatText = 'That is not a date.';
+      await tester.pumpWidget(inputDatePickerField(
+        errorFormatText: errorFormatText,
+        formKey: formKey,
+      ));
+      await tester.enterText(find.byType(TextField), '');
+      await tester.pumpAndSettle();
+      formKey.currentState!.validate();
+      await tester.pumpAndSettle();
+      expect(find.text(errorFormatText), findsOneWidget);
+    });
   });
 }