Set growth rate to NA if not available for latest R number date

[carol-davies]

Issue - See screenshot below, 6th July is the correct R number date for Northern Ireland but the growth rate is the last one that NI published which was on 17 November 2020. Northern Ireland no longer publish growth rates so this should be NA. It will also become an issue for Wales in the future as they will be publishing R numbers weekly but growth rates less frequently.

Preferred action - always publish the latest R number, but only include the growth rate if it is available for the same date, otherwise display growth rate as NA.