Fix unit tests failing on PHP nightly for the `WordPress.WP.I18n` sniff.
Fix unit tests failing on PHP nightly for the `WordPress.WP.I18n` sniff.

This solves the following error:
`An error occurred during processing; checking has been aborted. The error message was: "count(): Parameter must be an array or an object that implements Countable" at /home/travis/build/WordPress-Coding-Standards/WordPress-Coding-Standards/WordPress/Sniffs/WP/I18nSniff.php:233`

`array_shift()` returns null if the array is empty or not an array which may have happened when less arguments where passed to the i18n function then expected.
In that case `$tokens` will be null and start throwing the `countable` error for PHP nightly.

jrfnl/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards