WPDB Unit Tests: Fix a fatal error that occurs in the test_strip_invalid_text() test if the mbstring extension isn't activated in PHP.

git-svn-id: https://develop.svn.wordpress.org/trunk@30631 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/db/charset.php

@@ -76,6 +76,7 @@ class Tests_DB_Charset extends WP_UnitTestCase {
 			),
 		);
 
+		if ( function_exists( 'mb_convert_encoding' ) ) {
 			// big5 is a non-Unicode multibyte charset
 			$utf8 = "a\xe5\x85\xb1b"; // UTF-8 Character 20849
 			$big5 = mb_convert_encoding( $utf8, 'BIG-5', 'UTF-8' );
@@ -89,6 +90,7 @@ class Tests_DB_Charset extends WP_UnitTestCase {
 				'value'    => $big5,
 				'expected' => $big5
 			);
+		}
 
 		// The data above is easy to edit. Now, prepare it for the data provider.
 		$data_provider = $multiple = $multiple_expected = array();