Remove unnecessary null pointer checks

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Proposed new feature or change

[An extra null pointer check is not needed in functions](https://isocpp.org/wiki/faq/freestore-mgmt#delete-handles-null "Do I need to check for null before delete p?") like the following.
- [FilamentRenderToBuffer](https://github.com/isl-org/Open3D/blob/7eca0d5d9581d2599f8de39d5e42911b9de33d0c/cpp/open3d/visualization/rendering/filament/FilamentRenderToBuffer.cpp#L69 "Destructor for FilamentRenderToBuffer")
- [PythonUnlocker](https://github.com/isl-org/Open3D/blob/44b672625a0a18e636280e08288fe2aafad63673/cpp/pybind/visualization/gui/gui.cpp#L80 "Destructor for PythonUnlocker")

### References

_No response_

### Additional information

_No response_