ListVariables with an empty element list should do nothing (not throw IllegalStateException)

[ge0ffrey]

Instead it should just log that it couldn't do anything.

To reproduce, solve a ListVariable VRP (= quickstart) with an empty Visit list.

To see how it should behave - for consistency - solve employee scheduling with an empty shift list. Watch the DEBUG log.

[ge0ffrey]

There are unit tests for non-list variables around this.

[triceo]