Fix #80595: Resetting POSTFIELDS to empty array breaks request

Fix #80595: Resetting POSTFIELDS to empty array breaks request

This is mainly to work around https://github.com/curl/curl/issues/6455,
but not building the mime structure for empty hashtables is a general
performance optimization, so we do not restrict it to affected cURL
versions (7.56.0 to 7.75.0).

The minor change to bug79033.phpt is unexpected, but should not matter
in practice.

Closes GH-6606.