Always return a function

[Richienb]

If no strategy can be found, null shouldn't be returned as it can lead to bugs. Instead, next-tick should just call the callback.

[medikoo]

@Richienb thanks for proposal. Still I think that returning a function will give a false idea that tasks will be propagated to next event loop and that is likely to introduce bugs in systems that depend on such behavior

If in given setup it's ok to fallback to immediate execution I believe that such decision should be made explicitly, and it can be achieved easily as:

const eventuallyNextTick = require('next-tick') || cb => cb();