fix(multi-source): inject empty VALUES when a prior step matched nothing [release]

_inject_values_clause treated an empty accumulator the same as no
accumulator, returning the query unrestricted. A prior step that matched
nothing would then let the next step run against the full endpoint instead
of yielding zero results.

Distinguish the two cases: None (no prior step) leaves the query
unrestricted, while an empty list now injects an empty VALUES block so the
federated query correctly resolves to zero solutions.

