pure: guard against empty blocks

If blocks is empty, we append `None` to the returned list, which is
incorrect.

This subtle issue was caught by pytype, which correctly identified
the return value as List[Optional[Tuple]] because of this possibility.

Differential Revision: https://phab.mercurial-scm.org/D9279

mercurial/pure/bdiff.py

@@ -51,7 +51,10 b' def _normalizeblocks(a, b, blocks):'
                 shift += 1
         r.append((a1, b1, l1 + shift))
         prev = a2 + shift, b2 + shift, l2 - shift
-    r.append(prev)
+
+    if prev is not None:
+        r.append(prev)
+
     return r