debuginfod-client.c: Don't print empty line in header_callback

debuginfod-client.c: Don't print empty line in header_callback

libcurl passes an empty line to header_callback indicating the end
of the response's HTTP headers.

Currently this empty line is printed to the debuginfod_client's
verbose_fd with a "header" prefix:

    $ echo $DEBUGINFOD_URLS
    https://debuginfod.fedoraproject.org/
    $ debuginfod-find -vv debuginfo e2bbf033b548021c37866429f12a99bd33bd6e8d
    [...]
    header x-fedora-requestid: ZULLx0PPA8nmj8c8Hw-RtAACgAE
    header server: Apache
    header
    [...]

Prevent this unnecessary line of output by only printing non-empty
lines in header_callback.

Signed-off-by: Aaron Merey <amerey@redhat.com>