Coding Standards: Remove redundant check in wpdb::set_sql_mode().

This commit removes an unnecessary `empty( $modes_str )` check, as the previous code block already checks the same value and returns early.

Follow-up to [30587], [56475].

Props justlevine.
See #64238.

git-svn-id: https://develop.svn.wordpress.org/trunk@61243 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wpdb.php

@@ -960,13 +960,7 @@ public function set_sql_mode( $modes = array() ) {
 				return;
 			}
 
-			$modes_str = $modes_array[0];
-
-			if ( empty( $modes_str ) ) {
-				return;
-			}
-
-			$modes = explode( ',', $modes_str );
+			$modes = explode( ',', $modes_array[0] );
 		}
 
 		$modes = array_change_key_case( $modes, CASE_UPPER );