MultiSites.getAll returns wrong format under circumstances #5987

MultiSites.getAll should always return an array of objects but sometimes it returns only a simple array (if there is only one site that matches pattern I think).

Example Url:
http://demo2.piwik.org/index.php?module=API&method=MultiSites.getAll&period=year&date=today&format=JSON&token_auth=TOKEN&pattern=piw&enhanced=1

{
nb_visits: 0,
nb_actions: 0,
label: "Example Site",
visits_evolution: "0%",
actions_evolution: "0%",
...
}

should be wrapped in [...].

I tried to workaround this issue in Piwik Mobile but it does also not return the idSite if there is only one result which makes it impossible.

This leads to an "Invalid format error" in Piwik Mobile when using the search.