This is a tricky one, it used to work, but it was wrong, because in such
a scenario instead of passing NULL as required by the API, it would pass
a buffer containing the `\0` terminator.
This stopped working on a specific miniupnpc version, when they fixed
some network endianess issue on Windows, to which we made a workaround,
which in turn would probably result in failures when the interface is
specified.
This commit address the issue properly, by checking the specified
interface string size, and correctly passing NULL instead of the empty
string when necessary.
Also reverts the commit that introduced the bogus workaround:
e85330231c
One of those PR when the explaination is much longer then code changes
:).
