Themes: Avoid PHP warnings in the admin when no theme directory exists.

Props iseulde, chris_dev, swissspidy.
Fixes #35521.


git-svn-id: https://develop.svn.wordpress.org/trunk@40636 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/theme-install.php

@@ -28,6 +28,11 @@ if ( ! is_network_admin() ) {
 }
 
 $installed_themes = search_theme_directories();
+
+if ( false === $installed_themes ) {
+	$installed_themes = array();
+}
+
 foreach ( $installed_themes as $k => $v ) {
 	if ( false !== strpos( $k, '/' ) ) {
 		unset( $installed_themes[ $k ] );