fix: azure blob writes fail with permission mismatch when the SAS lacks the tag permission

[locle2302]

What does this PR do?

Passes nil instead of an empty azblob.BlobTagsMap{} to the azblob SDK in PutBlob, CommitBlockList, and StartCopyFromURL, so blob write requests no longer carry an empty x-ms-tags header.

Why is this PR needed?

The SDK's SerializeBlobTagsHeader only omits the x-ms-tags header when the tags map is nil; an empty map serializes to an empty header. Azure treats any request carrying x-ms-tags as an attempt to set blob tags, which requires the t permission on the SAS. The platform issues user-delegation SAS tokens with sp=racwdl, so every write through a goofys mount fails with 403 AuthorizationPermissionMismatch, surfacing as Permission denied on file close. Reads and lists are unaffected.

Steps to Reproduce

1. Mount an Azure container with a user-delegation SAS that has sp=racwdl (no t).
2. cp somefile /bucket/ — fails with failed to close: Permission denied; goofys logs show PutBlob rejected with AuthorizationPermissionMismatch.
3. Same 403 reproduces with plain curl by adding an empty x-ms-tags header to a PUT that otherwise succeeds; with this fix, writes through the mount succeed.