Change value property to fix #725 & #681 (#854)

According to documentation `initialValue` of any field can be null.

Author: ashim-kr-saha

lib/src/fields/form_builder_switch.dart

@@ -132,7 +132,7 @@ class FormBuilderSwitch extends FormBuilderField<bool> {
                 isThreeLine: false,
                 contentPadding: contentPadding,
                 title: title,
-                value: state.value!,
+                value: state.value ?? false,
                 onChanged: state.enabled
                     ? (val) {
                         state.requestFocus();