support zero length input in match.integer64

[hcirellu]

Zero length input for x and table to match.integer64 should result in consistent results as match with integer.

Closes #190

[MichaelChirico]

I finally understood how the R code works, it's somewhat unexpected:

https://github.com/r-devel/r-svn/blob/0f7760a26a9381915a52609bc18fd776eee23a41/src/main/coerce.c#L1884

The C-level coercion asInteger() always needs to return a single int value, so its behavior differs from as.integer() which returns integer() when encountering NULL.

[MichaelChirico]

Thanks!

Just minor points about GitHub itself:

• Issue numbers in PR titles are not very helpful since they don't convert into links as they do in PR descriptions: https://github.com/orgs/community/discussions/9369
• There are very specific ways to mention issue numbers that will let them close automatically when the PR is merged (see my edit above): https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword