handles empty args in backwards compatible functions

[jmbarbone]

Replaces list2(...) with dots_list(..., .ignore_empty = "all") to allow for empty, trailing args to be combined with other args

Adds test with example from #6710

This simple fix would allow for empty args placed in the middle (e.g., scale_fill_discrete(breaks = 1:2, , direction = -1L,) works fine). This may not be ideal, but some inject(scale_backward_compatibility(!!!list2(...), *)) worked fine in testing (and what I originally did in 6388981 because I forgot how dots_list() worked...)