Allow '' to be a valid key for Hash, and Session

[markstory]

By removing a bunch of empty() guards we can make '' behave like all the other key names. This does change the existing behavior/tests around '' key, but I think that is ok given the need to manipulate ''.

Refs #9632

[ADmad]

In what cases does using empty string as key actually make sense?

In reference to #9632 i would rather prevent empty string being used when saving cache. That way deleting it won't be even an issue.

[dereuromark]

@ADmad How can you prevent that for deeply nested array being written to the session?
You cannot traverse the whole tree when saving

$this->Session->write('Key', $veryDeepArray);

But you still need to read it when traversing over an array of those keys and in case there is some empty string in it:

foreach ($array as $v) {
    ->read('Key.foo.bar. ' $v ' .baz')
}

or sth?
I admit it is an edge case, but if it is possible to fix up rather than breaking it half way in a different direction with exceptions, I think that is the better way.

[ADmad]

Having an empty key is almost always an error but i see there's no easy way to prevent it. So i guesa the only option is to allow deleting such a key.