revlog: move `nullid` early return sooner in `_revisiondata`

Let us deal with the special case before we start dealing with more generic
case.

mercurial/revlog.py

@@ -1618,12 +1618,13 b' class revlog(object):'
             node = nodeorrev
             rev = None
 
+        if node == nullid:
+            return ""
+
         cachedrev = None
         flags = None
         rawtext = None
         basetext = None
-        if node == nullid:
-            return ""
         if self._revisioncache:
             if self._revisioncache[0] == node:
                 # _cache only stores rawtext