patchbomb: look for non-empty publicurl, not a non-None one

Otherwise it's impossible to turn this feature back off, which is
making writing of tests awkward.

hgext/patchbomb.py

@@ -113,7 +113,7 b' def _addpullheader(seq, ctx):'
     # waiting for some logic that check that the changeset are available on the
     # destination before patchbombing anything.
     publicurl = repo.ui.config('patchbomb', 'publicurl')
-    if publicurl is not None:
+    if publicurl:
         return ('Available At %s\n'
                 '#              hg pull %s -r %s' % (publicurl, publicurl, ctx))
     return None
@@ -588,7 +588,7 b' def email(ui, repo, *revs, **opts):'
 
     # check if revision exist on the public destination
     publicurl = repo.ui.config('patchbomb', 'publicurl')
-    if publicurl is not None:
+    if publicurl:
         repo.ui.debug('checking that revision exist in the public repo')
         try:
             publicpeer = hg.peer(repo, {}, publicurl)