Fix selects without empty_label were not allowed to be empty in some cases, even without required=true

For example select-account multiple=true, or customfield select-account with rows > 1 would give error "'' is NOT allowed ('not found')"

api/src/Etemplate/Widget/Select.php

@@ -171,7 +171,8 @@ class Select extends Etemplate\Widget
 			}
 			$allowed = array_map('strval', $allowed);
 
-			if (!$multiple || $this->attrs['multiple'] === "dynamic") $allowed[] = '';
+			// Add empty as an option, we check for required value later
+			$allowed[] = '';
 
 			foreach((array) $value as $val)
 			{