[@mantine/form] Fix error thrown for nullable values dirty status che…

…ck (#6672)

packages/@mantine/form/src/paths/get-path.ts

@@ -9,7 +9,7 @@ export function getPath(path: unknown, values: unknown): unknown {
 
   let value = values[splittedPath[0] as keyof typeof values];
   for (let i = 1; i < splittedPath.length; i += 1) {
-    if (value === undefined) {
+    if (value == null) {
       break;
     }