Replaces `null` returns from `SqlBreakdownBase.GetQuery<T>()` and its four
overrides (LinqQueryBreakdown, SqlServer/PostgreSql/Snowflake QueryBreakdown)
with `Enumerable.Empty<T>().AsQueryable()`, and tightens the signature from
`IQueryable<T>?` to `IQueryable<T>`. The "we can't reconstruct" semantic
now lives in "the query yields zero rows" rather than a nullable return,
which is what callers in LINQ pipelines actually want.
Three LinqQueryBreakdownTests tests asserting `Is.Null` are renamed and
updated to assert `Is.Empty`.
BREAKING CHANGE: SqlBreakdownBase.GetQuery<T> and the SqlServer / PostgreSql /
Snowflake / LinqToSql QueryBreakdown.GetQuery<T> overrides no longer return
`IQueryable<T>?`; they now return a non-nullable `IQueryable<T>` that is
empty when reconstruction isn't possible. External NuGet consumers null-
checking the result must switch to `.Any()` / `Is.Empty` checks instead.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
