Closes #632

The CURLOPT_SSH_PUBLIC_KEYFILE option has been documented to handle
empty strings specially since curl-7_25_0-31-g05a443a but the behavior
was unintentionally removed in curl-7_38_0-47-gfa7d04f.

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

for all we know now, it might be called 7.46.1

Without this workaround, NSS re-uses a session cache entry despite the
server name does not match.  This causes SNI host name to differ from
the actual host name.  Consequently, certain servers (e.g. github.com)
respond by 400 to such requests.

Bug: https://bugzilla.mozilla.org/1202264