Missing null check when parsing unverified SHA1

[mshandrovskiy]

https://github.com/Backblaze/b2-sdk-python/blob/4aa3d3c09cc0b847d4baad1f8247a68b3b606c66/b2sdk/_internal/file_version.py#L99

This function is invoked in context where content_sha1 is typed as str | None. This code will crash on None input. https://github.com/Backblaze/b2-sdk-python/blob/4aa3d3c09cc0b847d4baad1f8247a68b3b606c66/b2sdk/v2/file_version.py#L47

Simple test output with just related code copypasta into my own test runner.

Traceback (most recent call last):
  File "/Users/mikes/PycharmProjects/account-api/tests/testing.py", line 147, in wrapper
    return self.loop.run_until_complete(coro)
  File "/Users/mikes/.pyenv/versions/3.6.12/lib/python3.6/asyncio/base_events.py", line 488, in run_until_complete
    return future.result()
  File "/Users/mikes/PycharmProjects/account-api/tests/test_storage_providers.py", line 84, in test_b2_object_attr_sha1_unset
    result = await self.b2_provider.get_object_metadata('key', photo)
  File "/Users/mikes/PycharmProjects/account-api/accounts/files/v4_1/providers.py", line 167, in get_object_metadata
    if sha1.startswith(UNVERIFIED_CHECKSUM_PREFIX):
AttributeError: 'NoneType' object has no attribute 'startswith'