view .arcconfig @ 45863:68aedad4c11c

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

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