[BUGFIX] Restore optional nature of "label" argument

Allows using flux:field without label, causing the label
to be resolved from XLF file of provider extension.

Close: #1890

Classes/ViewHelpers/FieldViewHelper.php

@@ -25,7 +25,7 @@ public function initializeArguments()
     {
         $this->registerArgument('type', 'string', 'TCA field type', true);
         $this->registerArgument('name', 'string', 'Name of the attribute, FlexForm XML-valid tag name string', true);
-        $this->registerArgument('label', 'string', 'Label for field', true);
+        $this->registerArgument('label', 'string', 'Label for field');
         $this->registerArgument('exclude', 'bool', 'Set to FALSE if field is not an "exclude" field', false, false);
         $this->registerArgument('config', 'array', 'TCA "config" array', false, []);
         $this->registerArgument(