Coding Standards: Remove redundant check in get_terms().

The same conditional already calls `isset()`, which explicitly checks if the value is different from `null`.

Follow-up to [36614], [37599].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60445 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/taxonomy.php

@@ -1337,7 +1337,7 @@ function get_terms( $args = array(), $deprecated = '' ) {
 		$args['taxonomy'] = $taxonomies;
 	} else {
 		$args = wp_parse_args( $args, $defaults );
-		if ( isset( $args['taxonomy'] ) && null !== $args['taxonomy'] ) {
+		if ( isset( $args['taxonomy'] ) ) {
 			$args['taxonomy'] = (array) $args['taxonomy'];
 		}
 	}