FFM-10409 Mark targets as optional for variation methods

[erdirowlands]

What

We allow targets to to undefined when calling variation methods, but if "strictNullChecks": true is enabled in a Typescript project, then passing an undefined target will result in a compilation fail.
This marks the target as explicitly optional for all variation methods.

Testing

Manual:

1. with project that enabled strictNullChecks and passed undefined targets and real targets with targeting rules enabled on each flag type.
2. with project with no strictNullChecks for undefined and real targets with targeting rules enabled on each flag type.
3. Metrics post for each of the above scenarios correctly.

[knagurski]

Looks like you might have forgotten about the functions in index.ts