Fix NULL indexed value for LocalizationSetContentPickerFieldIndexHandler

[Skrypt]

Fixes #11248

See ContentPickerFieldIndexHandler (same principle) :

OrchardCore/src/OrchardCore.Modules/OrchardCore.ContentFields/Indexing/ContentPickerFieldIndexHandler.cs

Lines 13 to 29 in d782252

	if (field.ContentItemIds.Length > 0)
	{
	foreach (var contentItemId in field.ContentItemIds)
	{
	foreach (var key in context.Keys)
	{
	context.DocumentIndex.Set(key, contentItemId, options);
	}
	}
	}
	else
	{
	foreach (var key in context.Keys)
	{
	context.DocumentIndex.Set(key, "NULL", options);
	}
	}

[Piedone]

Probably my worst contribution ever to add Media indexing, it'll continue to haunt me forever :D.

[Skrypt]

Don't worry it is @jptissot fault on that one 🙈