fix round-trip compound ids with a null segment

[lsmith77]

Fixes #2716

RestApiHandler serializes a null segment of a compound id as an empty
string (makeCompoundId joins the segments), emitting ids like 200-01|
for a record whose optional id field is null. But coerce then runs that
empty segment through type coercion on parse (parseInt('') → NaN → 400),
so the handler rejects the very id it just emitted — any model with a
nullable field in its external/compound id has rows unaddressable by their
own id.

Fix: coerce gains an emptyAsNull flag, passed only from the id-parsing
paths (makeIdFilter, makeIdConnect). An empty segment for an optional
field now parses as null (matched via IS NULL), mirroring serialization.
Filter-value coercion is unchanged.

Known limitation: an empty trailing/middle segment is ambiguous — an optional String field with the actual value "" serializes identically to null, so on parse it always resolves to null. (Numeric/Boolean/etc. types are unaffected: "" was never a valid value there, so the only thing an empty segment can mean is null.) This PR picks "null wins" as the simplest unambiguous rule.

We could explore other options:

• global config
• model specific config
• OR matching both

Obviously the entire NULL case could also be rejected because it doesn't gurantee uniqueness.

[lsmith77]

I guess I will close this. NULLable columns should just not be part of a compound identifier.