Change hooks to check against null when no hook is specified

[evanplaice]

Currently, false is used as the value to indicate that a hook hasn't been set.

This can and will cause conflicts with user-defined hooks that use false as a return value for their particular use case.

This is an API breaking change so it should be included in the next major release.

Ref: #44

[evanplaice]

This change needs to be implemented because the current implementation will break in rare cases where the value being returned should be false