Fix `cacheThumbnail` if no $vcard was passed

This fixes a type confusion. `getContact` does not return a VCard-object, but an array containing either the VCard mapped to 'vcard' or the string representation of the VCard in 'carddata'.

Note that nowhere in the current codebase is `cacheThumbnail` called without the `$vcard` argument or an argument which could be `null`, so this is not strictly speaking a bug, but dead code which doesn't work.