revset: fix crash on empty sort key

Make it noop as before 2188f170f5b6. We could change it to an error, but
allowing empty key makes some sense for scripting that builds a key string
programmatically.

mercurial/revset.py

@@ -1890,6 +1890,8 b' def sort(repo, subset, x):'
                 'topo.firstbranch can only be used when using the topo sort '
                 'key'))
 
+    if not keys:
+        return revs
     if keys == ["rev"]:
         revs.sort()
         return revs

tests/test-revset.t

@@ -962,6 +962,13 b' test sorting two sorted collections in d'
   6
   2
 
+test empty sort key which is noop
+
+  $ log 'sort(0 + 2 + 1, "")'
+  0
+  2
+  1
+
 test invalid sort keys
 
   $ log 'sort(all(), -invalid)'