fix(plugin-version): Correctly check void releases for all falsy values

[martinjlowm]

js-yaml implements the YAML spec and it has no undefined type:

• How to dump empty key value to yaml? nodeca/js-yaml#356 (comment)

I.e.:

> require('js-yaml').safeLoad('')
undefined // if the file is empty
> require('js-yaml').safeLoad('releases:\n')
{ releases: null }

[arcanis]

Indeed! Probably should be != null then - we still want to cover the case where the releases field simply doesn't exist.

[martinjlowm]

Certainly, I don't know why I didn't realize the previous undefined check was for an absent key.

I've changed it to check for all falsy values (and an empty file which returns undefined, hence the optional operator), since I don't think '' nor 0 make sense either. I'm happy to make it less strict if you feel that is the better solution.

[arcanis]

Thanks!