Cosmos: Implement EF.Functions.CoalesceUndefined

[roji]

Cosmos has a coalescing operator (??); this coalesces Cosmos undefined (e.g. missing properties), but not null:

SELECT null ?? 'foo' -- returns null
SELECT undefined ?? 'foo' -- returns foo

As a result, this operator is unsuitable for translation of regular .NET coalescing operations; we already use the conditional operator for that (x != null ? x : 'foo'). However, the user should be able to express the coalescing operator via e.g. an EF.Functions.CoalesceUndefined() function.

This can probably be done together with #17722 (add IS_DEFINED() translation), as part of a better story around Cosmos undefined.

/cc @ajcvickers

[roji]

The coalescing operator is needed for certain translations, e.g. Where(b => b.Ints.FirstOrDefault() == 8, so I'll just go ahead and implement this.