typing: rewrite a conditional assignment to unconfuse pytype

Otherwise, pytype and PyCharm flags it:

    File "/mnt/c/Users/Matt/hg/mercurial/localrepo.py", line 2903, in wlock: No attribute 'held' on _weakref.ReferenceType[nothing] [attribute-error]
      In Optional[Union[Any, _weakref.ReferenceType[nothing]]]
    File "/mnt/c/Users/Matt/hg/mercurial/localrepo.py", line 2904, in wlock: No attribute 'lock' on _weakref.ReferenceType[nothing] [attribute-error]
      In Optional[Union[Any, _weakref.ReferenceType[nothing]]]

Differential Revision: https://phab.mercurial-scm.org/D10215

mercurial/localrepo.py

@@ -2891,7 +2891,7 b' class localrepository(object):'
 
         If both 'lock' and 'wlock' must be acquired, ensure you always acquires
         'wlock' first to avoid a dead-lock hazard."""
-        l = self._wlockref and self._wlockref()
+        l = self._wlockref() if self._wlockref else None
         if l is not None and l.held:
             l.lock()
             return l