chore: fix static analysis warnings

Author: danvick

packages/form_builder_extra_fields/lib/src/fields/form_builder_cupertino_date_time_picker.dart

@@ -187,9 +187,9 @@ class _FormBuilderCupertinoDateTimePickerState extends FormBuilderFieldState<
     super.initState();
     _textFieldController = widget.controller ?? TextEditingController();
     _dateFormat = widget.format ?? _getDefaultDateTimeFormat();
-    final _initialValue = initialValue;
+    final initialVal = initialValue;
     _textFieldController.text =
-        _initialValue == null ? '' : _dateFormat.format(_initialValue);
+        initialVal == null ? '' : _dateFormat.format(initialVal);
     effectiveFocusNode.addListener(_handleFocus);
   }
 

packages/form_builder_extra_fields/lib/src/fields/form_builder_signature_pad.dart

@@ -138,8 +138,8 @@ class _FormBuilderSignaturePadState
     _controller = widget.controller ?? SignatureController();
     _controller.addListener(() async {
       requestFocus();
-      final _value = await _getControllerValue();
-      didChange(_value);
+      final val = await _getControllerValue();
+      didChange(val);
     });
     SchedulerBinding.instance.addPostFrameCallback((Duration duration) async {
       // Get initialValue or if points are set, use the  points