tablegen should require at least one value field

[frolic]

A key-only table doesn't make much sense, because there's nothing to write to the record data if there are no value fields. You can't even check for record existence in that case, because you'd always get back zero bytes for an existing or non-existent key.

We can easily do this in tablegen, but also might be worth considering doing this in registerTable?

[qbzzt]

Alternatively, we can have a short cut that adds an exists: "bool" field.