document result of applying _.min or _.max to an empty list
Merge pull request #1619 from davidchambers/infinity

document result of applying _.min or _.max to an empty list
update _.has to return false for null/undefined

The current behaviour (throwing a TypeError) is not helpful, and likely
not deliberate.
Merge pull request #1624 from davidchambers/has

update _.has to return false for null/undefined