Bail out of 4.3 term splitting routine when no shared terms are found.

Props dd32.
See #30261.

git-svn-id: https://develop.svn.wordpress.org/trunk@32819 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/upgrade.php

@@ -1870,6 +1870,10 @@ function split_all_shared_terms() {
 		 HAVING term_tt_count > 1"
 	);
 
+	if ( empty( $shared_terms ) ) {
+		return;
+	}
+
 	// Rekey shared term array for faster lookups.
 	$_shared_terms = array();
 	foreach ( $shared_terms as $shared_term ) {