Handle empty dates

[zjwhitehead]

Currently when adding a date field to a template and calling a document that doesnt yet have that date filled out the SDK raises an error because it cant convert an empty string or nil to a Date/Time.
It will still raise an error for malformed dates.

This fixes it by returning an empty text fragment (so calling .value) still works.

I thought about a few other approaches but this seems to have the lowest impact. Open to feedback

[zjwhitehead]

Btw looks like these same tests that failed are failing in master

[zjwhitehead]

What are your thoughts on this @faresd ?

[faresd]

@alistairholt can you help here?

[alistairholt]

@faresd sure. I'll take a look.

[alistairholt]

@zjwhitehead thanks for your contribution and apologies for the massive delay in responding. While it works it feels slightly icky that we would return a Prismic::Fragments::Text instance when we are expecting a Prismic::Fragments::Date instance. I think it would be better if we adjust Prismic::Fragments::Date#to_html to simply do an early return with an empty string ('') if the value is nil.