ssh: make CURLOPT_SSH_PUBLIC_KEYFILE treat "" as NULL

The CURLOPT_SSH_PUBLIC_KEYFILE option has been documented to handle
empty strings specially since curl-7_25_0-31-g05a443ad but the behavior
was unintentionally removed in curl-7_38_0-47-gfa7d04fe.

This commit restores the original behavior and clarifies it in the
documentation that NULL and "" have both the same meaning when passed
to CURLOPT_SSH_PUBLIC_KEYFILE.

Bug: http://curl.haxx.se/mail/lib-2016-01/0072.html