chore(sonar)!: return empty IQueryable instead of null from GetQuery<T> (S1168)

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>

src/Strata.SqlTools.PostgreSql/Breakdowns/QueryBreakdown.cs

@@ -272,17 +272,13 @@ public class QueryBreakdown : SqlServerQueryBreakdown
     /// Gets a LINQ to SQL query of the specified type based on this breakdown.
     /// </summary>
     /// <typeparam name="T">The entity type for the query.</typeparam>
-    /// <returns>null by default, as QueryBreakdown operates on SQL. Override in derived classes to provide LINQ query reconstruction.</returns>
+    /// <returns>An empty queryable by default, as QueryBreakdown operates on SQL. Override in derived classes to provide LINQ query reconstruction.</returns>
     /// <remarks>
-    /// This PostgreSQL-specific implementation returns null since PostgreSQL QueryBreakdown represents parsed SQL statements.
+    /// This PostgreSQL-specific implementation returns an empty queryable since PostgreSQL QueryBreakdown
+    /// represents parsed SQL statements and has no built-in way to create LINQ queries.
     /// Derived classes can override this method to reconstruct LINQ queries from the analyzed components.
     /// </remarks>
-    public override IQueryable<T>? GetQuery<T>() where T : class
-    {
-        // PostgreSQL breakdown represents parsed SQL statements and does not have a built-in way to create LINQ queries
-        // Override in derived classes to provide LINQ query reconstruction if needed
-        return null;
-    }
+    public override IQueryable<T> GetQuery<T>() where T : class => Enumerable.Empty<T>().AsQueryable();
 }