Avoid eltype degrading to Union{} for empty map/broadcast

[c42f]

This reverts to using Core.Compiler.return_type for map/broadcast, but
only in the very restricted case that the output container is completely
empty.

This is consistent with the way that return_type is used in Base for
collect and broadcast for empty collections only.

Fixes #528 using the strategy from #528 (comment)

[c42f]

@tkoolen I think this finally fixes #528 in a way which is safe. Or at least, it's no worse than what Base already does for mapping an empty collection. If it's good enough for Base, I think it's good enough for us :-)

[tkoolen]

Thank you! Makes sense and looks good to me once tests pass.

[c42f]

Eh, that's a lot of test failures! That's what I get for pushing right after a last minute rebase. Lucky we have CI to keep me honest :-D Let's see what it says now.

[coveralls]

Coverage increased (+0.03%) to 81.561% when pulling 76d506e on cjf/fix-mapped-empty-eltype into ae78e52 on master.