gateway/azure: Return an empty list if no chunks uploaded yet

[pauln]

This makes ListParts behave more like S3 by returning an empty list rather than an error if no chunks/blocks have been uploaded yet.

This is a replacement for #5226, suggested by @krishnasrinivas in the comments on that PR.

Description

See #5226 for more background on and discussion about this fix.

Motivation and Context

As discussed in #5169, this is required for tus/tusd to be able to use Minio Azure Gateway.

How Has This Been Tested?

Tested manually, using tusd to send a repeatable and realistic set of calls which work as expected on S3 but don't currently work on Minio Azure Gateway.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added unit tests to cover my changes.
• I have added/updated functional tests in mint. (If yes, add mint PR # here: )
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #5230 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5230      +/-   ##
==========================================
+ Coverage   60.22%   60.22%   +<.01%     
==========================================
  Files         194      194              
  Lines       29472    29477       +5     
==========================================
+ Hits        17749    17753       +4     
- Misses      10392    10393       +1     
  Partials     1331     1331

Impacted Files	Coverage Δ
cmd/gateway-azure.go	21.31% <0%> (-0.16%)	⬇️
cmd/fs-v1-rwpool.go	60.86% <0%> (-4.35%)	⬇️
cmd/fs-v1-background-append.go	81.2% <0%> (+4.69%)	⬆️
cmd/xl-v1-common.go	100% <0%> (+5.12%)	⬆️

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[harshavardhana]

LGTM tested.