bug: `ibis.memtable([], schema={"a": "int64"}).execute()` errors, should give empty table

[NickCrews]

What happened?

If I do import ibis; ibis.memtable([], schema={"a": "int64"}).execute(), I would expect to get an empty table back. But it errors instead. Ibis DOES allow you to actually create the expression with ibis.memtable([], schema={"a": "int64"}), it only is when you .execute() it that the error happens.

If you agree with what the desired behavior should be, I can put up a PR.

What version of ibis are you using?

main

What backend(s) are you using, if any?

duckdb, but I think this should apply for all backends?

Relevant log output

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