Simplify CreateProvider method code

Author: AlexUstinov

Orm/Xtensive.Orm/Orm/Providers/SqlCompiler.Helpers.cs

@@ -35,17 +35,13 @@ protected SqlProvider CreateProvider(SqlSelect statement, IEnumerable<QueryParam
       CompilableProvider origin, params ExecutableProvider[] sources)
     {
       var allowBatching = true;
-      var parameterBindings = Enumerable.Empty<QueryParameterBinding>();
+      var parameterBindings = extraBindings ?? Enumerable.Empty<QueryParameterBinding>();
       foreach (var provider in sources.OfType<SqlProvider>()) {
         var queryRequest = provider.Request;
         allowBatching &= queryRequest.CheckOptions(QueryRequestOptions.AllowOptimization);
         parameterBindings = parameterBindings.Concat(queryRequest.ParameterBindings);
       }
 
-      if (extraBindings != null) {
-        parameterBindings = parameterBindings.Concat(extraBindings);
-      }
-
       var tupleDescriptor = origin.Header.TupleDescriptor;
 
       var options = QueryRequestOptions.Empty;