Upgrade/install: Improve logic check when determining installation st…

…atus.

Improve handling of ambiguous return values to determine if a blog is installed.

Props zieladam, xknown.
Merges [49377] to trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@49386 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -1756,7 +1756,8 @@ function is_blog_installed() {
 			continue;
 		}
 
-		if ( ! $wpdb->get_results( "DESCRIBE $table;" ) ) {
+		$described_table = $wpdb->get_results( "DESCRIBE $table;" );
+		if ( is_array( $described_table ) && count( $described_table ) === 0 ) {
 			continue;
 		}