Add Consitant Collection::getByHandle return value

* Collection::getByHandle() returns false if CacheLocal returns -1
* Collection::getByHandle() returns -1 if CacheLocal returns falsey
* Adds check before final return $ak to prevent returning -1 and instead
  return false

Main issue found with -1 return value is the Package::install() idiom
of `if(!CollectionAttributeKey::getByHandle('my_handle')) {` as
`!-1 === false`. This causes the installer to skip the installation of the
CollectionAttributeKey as it assumes it already installed.
Merge pull request #894 from mkly/collection_attribute_get_by_handle_return

Add Consitant CollectionAttributeKey::getByHandle return value