[FSDP] Fix accidental change in `_test_fsdp_parity` (#90252)

I accidentally changed the semantics of this line when refactoring a while ago. The [previous version](https://github.com/pytorch/pytorch/pull/80873/files#diff-7b5c66f99161fa6a3d9042e80f8c8cc140a64e43445feede46f55e53154f6c3dL635) used to say:
```
if not mixed_precision:
```
which is actually the opposite of
```
if mixed_precision is not None:
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90252
Approved by: https://github.com/zhaojuanmao
diff --git a/torch/testing/_internal/common_fsdp.py b/torch/testing/_internal/common_fsdp.py
index 0c40630..405d8a5 100644
--- a/torch/testing/_internal/common_fsdp.py
+++ b/torch/testing/_internal/common_fsdp.py
@@ -1029,7 +1029,7 @@
         # the DDP parameters are in FP16 (from `half()`) while the FSDP
         # parameters are in FP32 (from `summon_full_params()`) and (2) DDP runs
         # the optimizer in FP16 while FSDP runs it in FP32
-        if mixed_precision is not None:
+        if mixed_precision is None:
             self.assertEqual(
                 ddp_params,
                 fsdp_unsharded_params,