by_name() raises TypeError instead of returning None when nothing matches

[apdavison]

KGObject.by_name() raises TypeError rather than returning None when called without a client and no instance matches:

>>> from fairgraph.openminds.v4.controlled_terms import Species
>>> Species.by_name("definitely not a species")
TypeError: object of type 'NoneType' has no len()

The instance-library branch assigns the miss as None (fairgraph/kgobject.py:946):

if match == "equals":
    objects = cls._instance_lookup.get(name, None)

and line 954 then calls len(objects).

The equivalent openMINDS method returns None for the same input, so this is a regression introduced by fairgraph's override, and it affects only the no-client path.

It arises more often than "no such term" suggests, because the instance-library lookup is exact and case-sensitive: any difference in case is a miss, so Species.by_name("mus musculus") raises while Species.by_name("Mus musculus") returns the term.

Same shape as #115 — a None sentinel used without a guard. The fix is to treat a miss as an empty result; a regression test needs no KG connection.