Return 1 when all elements in l are positive.
abjad> listtools.sign([1, 2, 3])
1
Return -1 when all elements in l are negative.
abjad> listtools.sign([-1, -2, -3])
-1
Return 0 when l is empty.
abjad> listtools.sign([ ])
0
Otherwise, return None.
abjad> listtools.sign([1, 2, -3]) is None
True