Postgresql: Declare the same setting for nulls in indexes as in order by statement

otherwise, index order conflicts with order by which cause longer query execution.
According to documentation default settings for nulls are exact opposite.

Author: alex-kulakov

Orm/Xtensive.Orm.PostgreSql/Sql.Drivers.PostgreSql/v9_0/Translator.cs

@@ -30,6 +30,9 @@ public override void Translate(SqlCompilerContext context, object literalValue)
       }
     }
 
+    public override void TranslateSortOrder(IOutput output, bool ascending) =>
+      output.Append(ascending ? "ASC NULLS FIRST" : "DESC NULLS LAST");
+
     // Constructors
 
     public Translator(SqlDriver driver)