Switch back to better `not <list>` instead of `not len(<list>)` (#739)
Switch back to better `not <list>` instead of `not len(<list>)` (#739)

This was originally introduced when neighbors switched to returning a numpy
array in #719, but that change was reverted. Unfortunately it wasn't correctly
reverted in #710 too, so it ended up landing anyway. This undoes that mistake.
stellargraph/stellargraph,stellargraph/stellargraph