MySQL: Fix Pad operations

LPAD/RPAD functions of MySQL requre actual character, so explicitly
define whitespace as character.

Author: alex-kulakov

Orm/Xtensive.Orm.MySql/Sql.Drivers.MySql/v5_0/Compiler.cs

@@ -160,7 +160,7 @@ public override void Visit(SqlFunctionCall node)
           return;
         case SqlFunctionType.PadLeft:
         case SqlFunctionType.PadRight:
-          SqlHelper.GenericPad(node).AcceptVisitor(this);
+          SqlHelper.GenericPad(node, true).AcceptVisitor(this);
           return;
         case SqlFunctionType.Rand:
           SqlDml.FunctionCall(translator.TranslateToString(SqlFunctionType.Rand)).AcceptVisitor(this);

Orm/Xtensive.Orm/Sql/SqlHelper.cs

@@ -331,7 +331,7 @@ public static string TimeSpanToString(TimeSpan value, string format)
           days, hours, minutes, seconds, milliseconds);
     }
 
-    public static SqlExpression GenericPad(SqlFunctionCall node)
+    public static SqlExpression GenericPad(SqlFunctionCall node, bool padStringRequired = false)
     {
       string paddingFunction;
       switch (node.FunctionType) {
@@ -346,9 +346,12 @@ public static SqlExpression GenericPad(SqlFunctionCall node)
       }
       var operand = node.Arguments[0];
       var result = SqlDml.Case();
-      result.Add(
-        SqlDml.CharLength(operand) < node.Arguments[1],
-        SqlDml.FunctionCall(paddingFunction, node.Arguments));
+      var lenghtCheck = SqlDml.CharLength(operand) < node.Arguments[1];
+      var paddingItself = (padStringRequired && node.Arguments.Count < 3)
+        ? SqlDml.FunctionCall(paddingFunction, operand, node.Arguments[1], SqlDml.Literal(" "))
+        : SqlDml.FunctionCall(paddingFunction, node.Arguments);
+
+      _ = result.Add(lenghtCheck, paddingItself);
       result.Else = operand;
       return result;
     }