Fix curl headers when value is absent

[sbiscigl]

Description of changes:

Right now if a S3 request is made like

PutObjectRequest().WithMetadata({{"KeyWithValue", "value"}, {"KeyWithoutValue", ""}});

this will result in a curl request with headers like

[DEBUG] 2023-11-14 15:37:57.397 CURL [0x1de241b40] (HeaderOut) PUT /test.txt HTTP/1.1
Host: sbiscigl.s3.us-east-1.amazonaws.com
...
x-amz-meta-keywithvalue: value
...

without the header without value. after this change it will look like

[DEBUG] 2023-11-14 15:42:21.551 CURL [0x1de241b40] (HeaderOut) PUT /test.txt HTTP/1.1
...
x-amz-meta-keywithoutvalue:
x-amz-meta-keywithvalue: value
...

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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