Join sublists in list l by sign.
abjad> l = [[1, 2], [3, 4], [-5, -6, -7], [-8, -9, -10], [11, 12]]
abjad> t = listtools.join_sublists_by_sign(l)
[[1, 2, 3, 4], [-5, -6, -7, -8, -9, -10], [11, 12]]
abjad> l = [[1, 2], [], [], [3, 4, 5], [6, 7]]
abjad> t = listtools.join_sublists_by_sign(l)
[[1, 2], [], [3, 4, 5, 6, 7]]