Fix memory_limit type.

[ticdenis]

memory_limit must be null or string.

Q	A
Branch	master for features and deprecations
Bug fix?	yes/no
New feature?	yes/no
BC breaks?	yes/no
Deprecations?	yes/no
Documented?	yes/no
Fixed tickets	comma-separated list of tickets fixed by the PR, if any

New Task Checklist:

• Is the README.md file updated?
• Are the dependencies added to the composer.json suggestions?
• Is the doc/tasks.md file updated?
• Are the task parameters documented?
• Is the task registered in the tasks.yml file?
• Does the task contains phpspec tests?
• Is the configuration having logical allowed types?
• Does the task run in the correct context?
• Is the run() method readable?
• Is the run() method using the configuration correctly?
• Are all CI services returning green?

[veewee]

Thanks @ticdenis