[2.0.0-alpha3] Select widget source must be optional

Problem/Motivation

Select widget source must be optional.

Proposed resolution

/src/Plugin/UiPatterns/Source/SelectWidget.php
@@ -37,6 +37,7 @@ class SelectWidget extends SourcePluginPropValue {
       '#type' => 'select',
       '#default_value' => $this->getSetting('value'),
       "#options" => $options,
+      "#empty_option" => $this->t("- Select -"),
     ];