typing: declare the `_phasesets` member of `phasecache` to be `Optional`

Something in this area got flagged while making the repository class visible to
pytype (instead of being typed as `Any`).  A None assignment to something not
optional is wrong, and when I tried setting it to `{}` to keep it non-Optional,
some tests failed.  There are checks for the attr being None elsewhere, so this
seems to have just been an oversight.

mercurial/phases.py

@@ -387,7 +387,7 b' class phasecache:'
             self._phaseroots: Phaseroots = loaded[0]
             self.dirty: bool = loaded[1]
             self._loadedrevslen = 0
-            self._phasesets: PhaseSets = None
+            self._phasesets: Optional[PhaseSets] = None
 
     def hasnonpublicphases(self, repo: "localrepo.localrepository") -> bool:
         """detect if there are revisions with non-public phase"""