refactor(linq): extract GetEmptyQueryable<T> helper for empty-return paths

DRYs the four `Enumerable.Empty<T>().AsQueryable()` returns added in
`2c23ba4` (S1168 fix) into a single `public static IQueryable<T>
GetEmptyQueryable<T>()` helper on `LinqQueryBreakdown`. No behavior
change — all 1180 tests stay green.

Co-Authored-By: Thom Lamb <thomlamb@gmail.com>

src/Strata.SqlTools.LinqToSql/Breakdowns/LinqQueryBreakdown.cs

@@ -1,5 +1,5 @@
-using System.Linq.Expressions;
 using Strata.SqlTools.Breakdowns.SqlServer;
+using System.Linq.Expressions;
 using PostgreSqlBreakdown = Strata.SqlTools.Breakdowns.PostgreSql.QueryBreakdown;
 using SnowflakeBreakdown = Strata.SqlTools.Breakdowns.Snowflake.QueryBreakdown;
 
@@ -195,24 +195,32 @@ public class LinqQueryBreakdown : QueryBreakdown
     {
         if (OriginalExpression == null)
         {
-            return Enumerable.Empty<T>().AsQueryable();
+            return GetEmptyQueryable<T>();
         }
 
         try
         {
             if (OriginalExpression is Expression && EntityType == typeof(T))
             {
-                return Enumerable.Empty<T>().AsQueryable();
+                return GetEmptyQueryable<T>();
             }
 
-            return Enumerable.Empty<T>().AsQueryable();
+            return GetEmptyQueryable<T>();
         }
         catch
         {
-            return Enumerable.Empty<T>().AsQueryable();
+            return GetEmptyQueryable<T>();
         }
     }
 
+    /// <summary>
+    /// Get and empty queryable of T
+    /// </summary>
+    /// <typeparam name="T">The entity type for the query.</typeparam>
+    /// <returns>An empty IQueryable of the type.</returns>
+    public static IQueryable<T> GetEmptyQueryable<T>() where T : class
+        => Enumerable.Empty<T>().AsQueryable();
+
     /// <summary>
     /// Analyzes an INSERT operation for the given entity.
     /// </summary>