[Intl] Allow passing null as a locale fallback

Null is passed in update-data.php to prevent falling back to English locale during icu data import.

Author: jakzal

Locale.php

@@ -21,7 +21,7 @@
 final class Locale extends \Locale
 {
     /**
-     * @var string
+     * @var string|null
      */
     private static $defaultFallback = 'en';
 
@@ -31,24 +31,24 @@ final class Locale extends \Locale
      * The default fallback locale is used as fallback for locales that have no
      * fallback otherwise.
      *
-     * @param string $locale The default fallback locale
+     * @param string|null $locale The default fallback locale
      *
      * @see getFallback()
      */
-    public static function setDefaultFallback(string $locale)
+    public static function setDefaultFallback(?string $locale)
     {
         self::$defaultFallback = $locale;
     }
 
     /**
      * Returns the default fallback locale.
      *
-     * @return string The default fallback locale
+     * @return string|null The default fallback locale
      *
      * @see setDefaultFallback()
      * @see getFallback()
      */
-    public static function getDefaultFallback(): string
+    public static function getDefaultFallback(): ?string
     {
         return self::$defaultFallback;
     }