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.SqlBreakdown/Classes/SqlBreakdownBase.cs

@@ -57,13 +57,13 @@ public abstract class SqlBreakdownBase : ISqlBreakdown
     /// 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>An IQueryable of the specified type, or null if the breakdown cannot be converted to a LINQ query.</returns>
+    /// <returns>An IQueryable of the specified type; an empty queryable if the breakdown cannot be converted to a LINQ query.</returns>
     /// <remarks>
     /// This method allows derived breakdown classes to reconstruct or generate LINQ queries
     /// from the analyzed components (SELECT, WHERE, ORDER BY, etc.).
-    /// The default implementation returns null.
+    /// The default implementation returns an empty queryable.
     /// </remarks>
-    public virtual IQueryable<T>? GetQuery<T>() where T : class => null;
+    public virtual IQueryable<T> GetQuery<T>() where T : class => Enumerable.Empty<T>().AsQueryable();
 
     /// <summary>
     /// Gets the complete SQL query including optional setup and finish clauses.