Validate cached wrapper support for null cache keys

[chrispenny]

I was originally under the impression that providing a value of null to the <% cached %> wrapper was equivalent to using an <% uncached %> wrapper. It sounds like this might not be the case though.

Need to validate what the behaviour is of the <% cached %> wrapper is when a value of null is provided to it.

The workaround (for now) is to conditionally add/avoid the <% cached %> wrapper depending on whether $CacheKey returns a value or not.

[chrispenny]

It looks like $CacheCondition for the <% cached %> wrapper has been available since Silverstripe 4.7.

https://docs.silverstripe.org/en/4/developer_guides/templates/partial_template_caching

I think the solution would be for us to update the recommended installation docs to reflect this. Something like:

<% cached $CacheKey if $CacheKey %>

But then we should also add some sort of option in the docs for folks using < 4.7.