Fix empty character set parameter issue in mysqlcheck commands

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

Author: Copilot

features/db-check.feature

@@ -136,3 +136,29 @@ Feature: Check the database
     When I try `wp db check --no-defaults --debug`
     Then STDERR should match #Debug \(db\): Running shell command: /usr/bin/env (mysqlcheck|mariadb-check) --no-defaults %s#
 
+  Scenario: Empty DB credentials should not cause empty parameter errors
+    Given a WP install
+    And a wp-config.php file:
+      """
+      <?php
+      define( 'DB_NAME', 'wp_cli_test' );
+      define( 'DB_USER', 'wp_cli_test' );
+      define( 'DB_PASSWORD', 'password1' );
+      define( 'DB_HOST', 'localhost' );
+      define( 'DB_CHARSET', '' );
+      define( 'DB_COLLATE', '' );
+      $table_prefix = 'wp_';
+      require_once ABSPATH . 'wp-settings.php';
+      """
+
+    When I run `wp db check --debug`
+    Then STDERR should not contain:
+      """
+      --default-character-set=
+      """
+    And the return code should be 0
+    And STDOUT should contain:
+      """
+      Success: Database checked.
+      """
+

src/DB_Command.php

@@ -1827,6 +1827,15 @@ private static function run( $cmd, $assoc_args = [], $send_to_shell = true, $int
 			unset( $assoc_args['dbpass'], $assoc_args['password'] );
 		}
 
+		// Filter out empty string values to avoid passing empty parameters to MySQL commands
+		// which can cause errors like "Character set '' is not a compiled character set"
+		$required = array_filter(
+			$required,
+			static function ( $value ) {
+				return '' !== $value;
+			}
+		);
+
 		$final_args = array_merge( $required, $assoc_args );
 
 		// Adapt ordering of arguments.