NSFS | close stream when getting empty content dir

[nadavMiz]

Explain the changes

1. currently we don't close the write stream before returning from read_object_md for empty content dir case. this causes the client to wait for the stream to close until it times out. added closing of the stream in that case

Issues: Fixed #xxx / Gap #xxx

Testing Instructions:

manual:

1. put empty content dir object: s3api put-object --bucket test-bucket --key key5/
2. get the new object: 's3api get-object --bucket test-bucket --key key5 outfile`
3. get should succeed with empty file

automatic:
sudo NC_CORETEST=true node ./node_modules/mocha/bin/mocha src/test/unit_tests/test_namespace_fs.js

• Doc added/updated
• Tests added

[shirady]

LGTM
minor comments