client: use default CA store during client auth if serverca is unset

Prior to this change, if the following conditions were true:

1) A Koji client uses SSL authentication,
2) The user does not set the "serverca" option for their profile,
3) The user does not have a "~/.koji/serverca.crt" file present,

Then Koji did not use the default CA store to validate the hub's CA.
read_defaults() was setting the "serverca" value to an empty string ""
in these cases. This led to an AuthError in ssl_login() for the empty
string:

  raise AuthError("Server CA %s doesn't exist or is not accessible" % serverca)

Update ssl_login() to ignore empty strings in this case, treating them
the same as None.

The code in _sendOneCall() already checks this value in a similar way,
with "if verify:", so we're matching that behavior here.

With this change, Koji clients will fall back to using the default CA
store.

koji/__init__.py

@@ -2583,7 +2583,7 @@ class ClientSession(object):
             raise AuthError('No certification provided')
         if not os.access(cert, os.R_OK):
             raise AuthError("Certificate %s doesn't exist or is not accessible" % cert)
-        if serverca is not None and not os.access(serverca, os.R_OK):
+        if serverca and not os.access(serverca, os.R_OK):
             raise AuthError("Server CA %s doesn't exist or is not accessible" % serverca)
         # FIXME: ca is not useful here and therefore ignored, can be removed
         # when API is changed