chore(validators): validate email if value is null

Author: danvick

packages/form_builder_validators/lib/src/form_builder_validators.dart

@@ -151,7 +151,7 @@ class FormBuilderValidators {
     String? errorText,
   }) =>
       (valueCandidate) =>
-          true == valueCandidate?.isNotEmpty && !isEmail(valueCandidate ?? '')
+          (valueCandidate?.isNotEmpty ?? false) && !isEmail(valueCandidate!)
               ? errorText ?? FormBuilderLocalizations.current.emailErrorText
               : null;