hook_requirements() seems to need a value

If panels is functioning correctly and not in legacy mode, it returns an entry from `panels_requirements('runtime')` that does not have a value. In spite of the `hook_requirements()` asserting that the `value` member of the requirements structure is not required, I get a Notice due to the missing field when displaying the status report.

I could just ignore it or suppress notices, but I prefer the attached patch.