Update FirebirdOrmLiteDialectProvider.cs

fixed: 
method ToUpdateRowStatement -> updateField=null

Author: angelcolmenares

src/ServiceStack.OrmLite.Firebird/FirebirdOrmLiteDialectProvider.cs

@@ -233,8 +233,11 @@ public override string ToInsertRowStatement(IDbCommand dbCommand, object objWith
 		}
 
 
-		public override string ToUpdateRowStatement(object objWithProperties, ICollection<string> updateFields)
+		public override string ToUpdateRowStatement(object objWithProperties, ICollection<string> updateFields=null)
 		{
+			if (updateFields == null) 
+				updateFields = new List<string>();
+				
 			var sqlFilter = new StringBuilder();
 			var sql = new StringBuilder();
 			var tableType = objWithProperties.GetType();