gnutls: Handle empty (but not NULL) passwords on PKCS#12 certs
gnutls: Handle empty (but not NULL) passwords on PKCS#12 certs

GnuTLS can accept PKCS#12 certs with either empty or NULL passwords[1],
but unlike OpenSSL[2], both options need to be tried separately.

[1] https://gitorious.org/gnutls/gnutls/commit/7c4c21c0e84a539558e3e1689d8a7b8a2c4c0056
[2] http://git.openssl.org/gitweb/?p=openssl.git;a=commitdiff;h=a331a305e9c9c5353bd42db6dbda78a418285708

Signed-off-by: Kevin Cernekee <3105dbb3dd63fbb4f6371cfb04adc6726f76c56b@gmail.com>

nmav/openconnect-mine,mtmiller/openconnect,nmav/openconnect-mine,chadcatlett/openconnect,mtmiller/openconnect,neshema/openconnect,atyndall/openconnect,lins05/openconnect,mveplus/openconnect-client,chadcatlett/openconnect,lins05/openconnect,matlockx/openconnect,chadcatlett/openconnect,matlockx/openconnect,liiir1985/openconnect,don-johnny/openconnect,mveplus/openconnect-client,cernekee/openconnect,neshema/openconnect,don-johnny/openconnect,brandt/openconnect,cernekee/openconnect,mveplus/openconnect-client,matlockx/openconnect,brandt/openconnect,brandt/openconnect,lins05/openconnect,matlockx/openconnect,cernekee/openconnect,atyndall/openconnect,brandt/openconnect,mtmiller/openconnect,cernekee/openconnect,neshema/openconnect,lins05/openconnect,chadcatlett/openconnect,don-johnny/openconnect,liiir1985/openconnect,liiir1985/openconnect,nmav/openconnect-mine,mveplus/openconnect-client,atyndall/openconnect,liiir1985/openconnect,atyndall/openconnect,mtmiller/openconnect,don-johnny/openconnect,nmav/openconnect-mine,neshema/openconnect