Fix "Undefined offset: 1" exception in Diviner when @task value in PHPDoc is a single word

Summary:
The PHPDoc in https://we.phorge.it/source/phorge/browse/master/src/infrastructure/storage/lisk/LiskMigrationIterator.php$14 defines * @task storage.
That means the value of $task in the DivinerAtomController class is the single string storage and there is no whitespace to explode on and there is no second key in the array to set as $title.
Thus only call explode when $task contains a whitespace.

EXCEPTION: (RuntimeException) Undefined array key 1 at [<arcanist>/src/error/PhutilErrorHandler.php:273]
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diviner/controller/DivinerAtomController.php:450]

Closes T15854

Test Plan: Go to https://we.phorge.it/book/contrib/article/database/#primary-keys and click the LiskMigrationIterator link pointing to https://we.phorge.it/book/dev/class/LiskMigrationIterator/

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15854

Differential Revision: https://we.phorge.it/D25690