fix: empty chunks not being uploaded correctly with GCP

OpenVDS has to be able to upload empty chunks. It is part of the VDS
data specification that empty chunks exists as an object on cloud
storage. Currently the code for GCP can not upload it because the
http_request code assumes it is a file and not a data chunk with a
header and footer.
16 jobs for !114 with jorgen.lind/gcp_empty_chunks in 22 minutes and 23 seconds (queued for 5 seconds)
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