Cannot read property 'charCodeAt' of null when saving untitled file in Makefile mode (fixes #11395)

src/vs/base/common/mime.ts

@@ -252,5 +252,5 @@ export function suggestFilename(theMime: string, prefix: string): string {
 		}
 	}
 
-	return null;
+	return prefix; // without any known extension, just return the prefix
 }
\ No newline at end of file