typing hint in _deserialize (image.py)

[csabaszeles]

Added type hinting for None type return as well, as PyCharm highlights it as well:
-> Union[None, FileStorage]

[jslvtr]

Thank you for this! I was wondering about Optional[FileStorage] as an alternative but after considering it carefully, it reads better (to me) that Optional be used only in the parameters, and not in the return value.

[jslvtr]

Cancelling approval of this PR since the changes need to be propagated to the rest of the code (e.g. section 6, lecture 81, start and end folders, as well as every other place where -> FileStorage is used.