Have /decide include disabled flag keys mapping to null

[havenbarnes]

Our docs mention that disabled flags will return as either null or undefined (https://posthog.com/docs/feature-flags/creating-feature-flags#enable-feature-flag). In reality /decide will simply not include these keys at all (which is technically equivalent to undefined after HTTP transport). It would be more useful for clients to see these keys mapped to null every time, so that SDK users can differentiate between a disabled flag and non-existent flag / posthog error response.

[benjackwhite]

One thing to consider - people might purposefully want it this way. So that they don't show flag keys that the user shouldn't have access to.

I can't think of a super strong reason why but simply showing all keys might not be what's wanted 🤷

Also many older SDKs don't check the value, they just grab the keys instead. Would need to be sure to make this a version change to the api (or maybe even just make this part of the new flags service instead)