Allow empty input table in ast compute_column

[wence-]

Description

If the input table is empty we can still produce an output of
appropriate type by letting the parser run and inferring types.
In this case though, we do not want to actually run a kernel,
since we would try and launch with zero blocks which is not
allowed.

Closes #13236.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

Thanks for the patch!

[wence-]

/merge