#977: Reject empty if_absent matching criteria

[gemshrine]

Fixes #977

Cause

Fbe.if_absent builds its lookup query from the callback's attributes after excluding system fields. With no remaining attributes, it sends (and ) to Factbase. That query can match an unrelated existing fact, and an empty factbase can instead receive a blank fact.

Change

The method now raises Fbe::Error before querying or inserting when the callback sets no non-system attributes. The same guard covers callbacks that set only _id, _time, or _version.

Test coverage

Regression cases cover an empty callback with an unrelated fact already present and a callback that sets only _id. Run them with bundle exec ruby -Itest test/fbe/test_if_absent.rb; repository CI also runs the full Rake test suite.