Selection::getPreviousAccessedColumns must return array or null [Closes #160]

[hranicka]

• bug fix? yes
• new feature? no
• BC break? no

Just fixed annotation + array return typehint for Selection::getPreviousAccessedColumns.

Fixes #160

[dg]

What about change FALSE to NULL via return array_keys(array_filter((array) $this->previousAccessedColumns)) ?: NULL? It will be consistent with other similar changes in framework.

[hranicka]

@dg Right, it's fine. I've changed it, master :-)

[hranicka]

Now, tests are failing. Previous changes (return an empty array instead of FALSE) were OK: https://travis-ci.org/nette/database/builds/197590334