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)

~~Simplify read_defaults() to only expand "cert" and "serverca" if the default files exist.~~

~~With this change, the SSL authentication code will fall back to the system-wide CA bundle instead of raising AuthError in ssl_login(). This matches the behavior with GSSAPI authentication.~~

EDIT after discussion below:
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.