Allow empty string to be passed to file_getdirlist

This allows the open/save dialogs to work in the editor when a filename without a path separator is passed (which is the default)

common/ui/file.cpp

@@ -41,7 +41,7 @@ static PHYSFSX_counted_list file_getdirlist(const char *dir)
 {
 	ntstring<PATH_MAX - 1> path;
 	auto dlen = path.copy_if(dir);
-	if (!dlen || !path.copy_if(dlen, "/"))
+	if ((!dlen && strlen(dir) > 0) || !path.copy_if(dlen, "/"))
 		return nullptr;
 	++ dlen;
 	PHYSFSX_counted_list list{PHYSFS_enumerateFiles(dir)};