Replace Empty IN() with IN(NULL)

Author: mythz

src/ServiceStack.OrmLite/Expressions/SqlExpression.cs

@@ -556,8 +556,15 @@ private string FormatFilter(string sqlFilter, params object[] filterParams)
 
                 if (filterParam is SqlInValues sqlParams)
                 {
-                    var sqlIn = CreateInParamSql(sqlParams.GetValues());
-                    sqlFilter = sqlFilter.Replace(pLiteral, sqlIn);
+                    if (sqlParams.Count > 0)
+                    {
+                        var sqlIn = CreateInParamSql(sqlParams.GetValues());
+                        sqlFilter = sqlFilter.Replace(pLiteral, sqlIn);
+                    }
+                    else 
+                    {
+                        sqlFilter = sqlFilter.Replace(pLiteral, SqlInValues.EmptyIn);
+                    }
                 }
                 else
                 {

src/ServiceStack.OrmLite/SqlInValues.cs

@@ -4,6 +4,8 @@ namespace ServiceStack.OrmLite
 {
     public class SqlInValues
     {
+        public const string EmptyIn = "NULL";
+        
         private readonly IEnumerable values;
         private readonly IOrmLiteDialectProvider dialectProvider;
 
@@ -24,7 +26,7 @@ public SqlInValues(IEnumerable values, IOrmLiteDialectProvider dialectProvider=n
         public string ToSqlInString()
         {
             return Count == 0 
-                ? "NULL" 
+                ? EmptyIn
                 : OrmLiteUtils.SqlJoin(values, dialectProvider);
         }