changelog: make copies related function return None or a valid value

With the previous code, existing but empty value were not "decoded", leading to
the method returning one of `None`, some valid value (`list` or `dict`) or
`b''`.

On a general basis, not explicitly checking for None is a source of bugs.
Having a clean return types will help the side-data copies code in future
changesets.

Differential Revision: https://phab.mercurial-scm.org/D7037

mercurial/changelog.py

@@ -363,22 +363,30 b' class changelogrevision(object):'
     @property
     def filesadded(self):
         rawindices = self.extra.get(b'filesadded')
-        return rawindices and decodefileindices(self.files, rawindices)
+        if rawindices is None:
+            return None
+        return decodefileindices(self.files, rawindices)
 
     @property
     def filesremoved(self):
         rawindices = self.extra.get(b'filesremoved')
-        return rawindices and decodefileindices(self.files, rawindices)
+        if rawindices is None:
+            return None
+        return decodefileindices(self.files, rawindices)
 
     @property
     def p1copies(self):
         rawcopies = self.extra.get(b'p1copies')
-        return rawcopies and decodecopies(self.files, rawcopies)
+        if rawcopies is None:
+            return None
+        return decodecopies(self.files, rawcopies)
 
     @property
     def p2copies(self):
         rawcopies = self.extra.get(b'p2copies')
-        return rawcopies and decodecopies(self.files, rawcopies)
+        if rawcopies is None:
+            return None
+        return decodecopies(self.files, rawcopies)
 
     @property
     def description(self):