manifest: don't store None in fulltextcache

When we read a value from fulltextcache, we expect it to be an array,
so we should not store None in it. Found while working on narrowhg.

mercurial/manifest.py

@@ -1210,6 +1210,7 b' class manifestrevlog(revlog.revlog):'
                 n = self.addrevision(text, transaction, link, p1, p2)
                 arraytext = array.array('c', text)
 
+        if arraytext is not None:
         self.fulltextcache[n] = arraytext
 
         return n
@@ -1506,6 +1507,7 b' class manifest(manifestrevlog):'
             m = self._newmanifest(text)
             arraytext = array.array('c', text)
         self._mancache[node] = m
+        if arraytext is not None:
         self.fulltextcache[node] = arraytext
         return m