empty list value for Schedule fields is undefined

[njr-11]

The Schedule annotation does not define what to do when fields (such as months or hours) are assigned a value of empty list.

One option could be for empty to indicate that it should run at no hours or no months, but that would be pointless to use the annotation at all in that case.

Or empty could raise an error.

Or another option would be for empty to indicate that the field is disregarded entirely, effectively being able to run at all of the values. This seems the most useful because it could be more convenient than listing all of the possible values.

The pull updates the JavaDoc to define the behavior as the latter. Also, I noticed the JavaDoc for these methods was difficult to read with everything in a single paragraph, so I split it up into paragraphs.

[KyleAure]

I agree that using an empty list to disregard is the correct choice.