Allow site enabledFrom and enabledTo dates to be null.

This code change allows a site's enabledFrom and enabledTo dates to be null.

It may be desirable to not have to worry about the enabledFrom and enabledTo dates at all, rather than having to extend them every 5 years, when everyone involved with the site has likely forgotten about the need to extend the enabledTo date.

This pull request was made with an updated master, and is therefore a lot less likely to be confusing to read than the previous one I submitted and subsequently closed.

Thanks!