Handle null preconditions correctly and simplify gcs lib again

[tylerwilliams]

No description provided.

[jdhollen]

from the gcs docs:

	// If ChunkSize is set to zero, chunking will be disabled and the object will
	// be uploaded in a single request without the use of a buffer. This will
	// further reduce memory used during uploads, but will also prevent the writer
	// from retrying in case of a transient error from the server or resuming an
	// upload that fails midway through, since the buffer is required in order to
	// retry the failed request.

is there some higher-level retry logic i'm missing to compensate for this? (it didn't look like it to me, but i'm also not sure we should care?)

[tylerwilliams]

I didn't add anything, but given that we're already disabling this for chunks under 16M (most chunks) it seems fairly safe. LMK if you disagree tho, not attached to this part if you feel strongly.