blob/azblob: Pass nil instead of an empty map for tags on Copy operation

[leezen]

Fixes #2982

Providing an empty map ends up causing the underlying SDK to want to have permissions to write tags against the destination blob. However, we don't actually need this permission since we're not really writing any tags. Instead, we should just pass in nil.

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[vangent]

nit: BlobTagsMap (typo)

[leezen]

Haha, not sure how I mistyped that. Guess I must be thinking about blogs...

[vangent]

Hmm this looks like it's a change to the golden file, which is hard for you to do. If you don't mind I'll clone it and send it to you.

[vangent]

Cloned into #2989.

[leezen]

@vangent Sounds good.