transfer: fix sendrecv() without interim poll

transfer: fix sendrecv() without interim poll

There was a "clever" optimization that skipped sendrecv() handling when
the transfer's pollset was empty. This happens for paused transfers, for
example.

Unforunately, if the libcurl application never calls curl_multi_poll(),
the pollset is and will aways remain empty, prevent the transfer from
progressing.

Remove this "optimization" and always try send/receive where applicable.

Fixes #14898
Reported-by: Victor Kislov
Closes #14901