PHPStan: Fix level 6 errors

adminer/include/coverage.inc.php

@@ -3,6 +3,7 @@ namespace Adminer;
 
 // coverage is used in tests and removed in compilation
 if (extension_loaded("xdebug") && file_exists(sys_get_temp_dir() . "/adminer.coverage")) {
+	/** @return void */
 	function save_coverage() {
 		$coverage_filename = sys_get_temp_dir() . "/adminer.coverage";
 		$coverage = unserialize(file_get_contents($coverage_filename));