fix NPE in Path.Combine if binPath isn't set

in some special environments (e.g. game engines) AppDomain BaseDirectory isn't set.
Workaround: new Configuration().SetProperty(
                "nhibernate-logger",
                typeof(Log4NetLoggerFactory).AssemblyQualifiedName
            )
Merge pull request #224 from childnodeContribArch/patch-1

NH-3540: fix NPE in Path.Combine if binPath isn't set