Only add _ComputeRepositoryURL if svnurl is not None

Thanks to bdbaddog for reporting, and to tomprince for the fix

master/buildbot/steps/source.py

@@ -565,7 +565,7 @@ def __init__(self, svnurl=None, baseURL=None, defaultBranch=None,
             warn("Please use workdir=, not directory=", DeprecationWarning)
             kwargs['workdir'] = directory
 
-        self.svnurl = _ComputeRepositoryURL(svnurl)
+        self.svnurl = svnurl and _ComputeRepositoryURL(svnurl)
         self.baseURL = _ComputeRepositoryURL(baseURL)
         self.branch = defaultBranch
         self.username = username