[ocs API] getData() always needs to return an array

As discussed in this issue getData() always needs to return an array: #6745 

Otherwise we will get this kind of warnings:
```
Warning:  array_merge_recursive(): Argument #2 is not an array in /home/src/owncloud/web/oc/lib/private/api.php on line 170
```

I tested it with the OCS Share API successfully.

cc @tomneedham please also have a look at it, thanks!