Address code review feedback: handle null values and improve test

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

features/db-check.feature

@@ -152,7 +152,15 @@ Feature: Check the database
       """
 
     When I run `wp db check --debug`
-    Then STDERR should not contain:
+    Then STDERR should contain:
+      """
+      Debug (db): Final MySQL command:
+      """
+    And STDERR should not contain:
+      """
+      --default-character-set=''
+      """
+    And STDERR should not contain:
       """
       --default-character-set=
       """

src/DB_Command.php

@@ -1832,7 +1832,7 @@ private static function run( $cmd, $assoc_args = [], $send_to_shell = true, $int
 		$required = array_filter(
 			$required,
 			static function ( $value ) {
-				return '' !== $value;
+				return null !== $value && '' !== $value;
 			}
 		);