Storage: Possible metadata regression on blobs in 1.24.0

[HenrikOssipoff]

Environment details

• Debian 10
• google-cloud-storage version: 1.24.1

Steps to reproduce

It's documented that metadata that isn't set will return a NoneType. Naturally, when wanting to unset any metadata, you expect to be able to pass in None. This worked up until 1.24.0 which broke this with googleapis/google-cloud-python#9796.

I think you can still technically set the metadata to an empty dictionary and get the same functionality, but it's a bit counter intuitive, when no metadata is described as being a NoneType.

Code example

storage_client = storage.Client(...)
blob = storage_client.get_bucket('abc').get_blob('abc')
blob.metadata = None
blob.patch()

Stack trace

AttributeError: 'NoneType' object has no attribute 'items'
  File "django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "rest_framework/views.py", line 505, in dispatch
    response = self.handle_exception(exc)
  File "rest_framework/views.py", line 465, in handle_exception
    self.raise_uncaught_exception(exc)
  File "rest_framework/views.py", line 476, in raise_uncaught_exception
    raise exc
  File "rest_framework/views.py", line 502, in dispatch
    response = handler(request, *args, **kwargs)
  File "api/media/views.py", line 96, in put
    product.media.set_primary(name)
  File "media/api.py", line 134, in set_primary
    blob.metadata = None
  File "/usr/local/lib/python3.7/site-packages/google/cloud/storage/blob.py", line 1917, in metadata
    value = {k: str(v) for k, v in value.items()}