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>

tests/Strata.SqlTools.LinqToSql.Tests/LinqQueryBreakdownTests.cs

@@ -513,21 +513,21 @@ public class GetQueryTests
     }
 
     [Test]
-    public void GetQuery_LinqBreakdown_ReturnsNullByDefault()
+    public void GetQuery_LinqBreakdown_ReturnsEmptyByDefault()
     {
         // Arrange
         var query = TestDataContext.Users.Where(u => u.Age > 18);
         var breakdown = LinqQueryBreakdown.Analyze(query);
 
-        // Act - GetQuery returns null because LinqQueryBreakdown needs the original provider
+        // Act - GetQuery returns empty because LinqQueryBreakdown needs the original provider
         var result = breakdown.GetQuery<User>();
 
         // Assert
-        Assert.That(result, Is.Null);
+        Assert.That(result, Is.Empty);
     }
 
     [Test]
-    public void GetQuery_SqlServerQueryBreakdown_ReturnsNull()
+    public void GetQuery_SqlServerQueryBreakdown_ReturnsEmpty()
     {
         // Arrange
         var breakdown = new QueryBreakdown("Id, Name", "Users", "Age > 18");
@@ -536,11 +536,11 @@ public class GetQueryTests
         var result = breakdown.GetQuery<User>();
 
         // Assert
-        Assert.That(result, Is.Null);
+        Assert.That(result, Is.Empty);
     }
 
     [Test]
-    public void GetQuery_MultipleBreakdownTypes_AllReturnNull()
+    public void GetQuery_MultipleBreakdownTypes_AllReturnEmpty()
     {
         // Arrange
         var linqBreakdown = LinqQueryBreakdown.Analyze(TestDataContext.Users);
@@ -553,8 +553,8 @@ public class GetQueryTests
         Assert.Multiple(() =>
         {
             // Assert
-            Assert.That(linqResult, Is.Null);
-            Assert.That(sqlResult, Is.Null);
+            Assert.That(linqResult, Is.Empty);
+            Assert.That(sqlResult, Is.Empty);
         });
     }