[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
