Improve check if arrows are empty
Improve check if arrows are empty

Prevents a numpy error if an empty array is given.

Eric89GXL/vispy,michaelaye/vispy,ghisvail/vispy,michaelaye/vispy,Eric89GXL/vispy,michaelaye/vispy,drufat/vispy,ghisvail/vispy,drufat/vispy,Eric89GXL/vispy,ghisvail/vispy,drufat/vispy