[Vulkan] Fix mm op null pointer dereference on Android (#79701)
Summary: The use of `c10::nullopt` instead of `c10::optional<Tensor>()` caused a null pointer dereference on Android.
Test Plan:
On Mac:
```
buck run //xplat/caffe2:pt_vulkan_api_test_binAppleMac
```
On Android:
```
buck build -c ndk.custom_libcxx=false -c pt.enable_qpl=0 //xplat/caffe2:pt_vulkan_api_test_binAndroid\#android-arm64 --show-output
adb push buck-out/gen/xplat/caffe2/pt_vulkan_api_test_binAndroid\#android-arm64 /data/local/tmp/vulkan_api_test
adb shell "/data/local/tmp/vulkan_api_test"
```
Reviewed By: SS-JIA
Differential Revision: D37189192
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79701
Approved by: https://github.com/SS-JIA
