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

[vangent]

Cloned from #2988 by @leezen, because it requires updating some golden files

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.

[leezen]

@vangent Thanks for bringing this over!

[codecov]

Codecov Report

Merging #2989 (1488303) into master (7b4db4c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2989      +/-   ##
==========================================
+ Coverage   69.55%   69.57%   +0.01%     
==========================================
  Files         111      111              
  Lines       11615    11614       -1     
==========================================
+ Hits         8079     8080       +1     
+ Misses       2867     2865       -2     
  Partials      669      669              

Impacted Files	Coverage Δ
blob/azureblob/azureblob.go	77.71% <100.00%> (-0.05%)	⬇️
blob/s3blob/s3blob.go	89.47% <0.00%> (+0.52%)	⬆️

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