Handle empty polylines

[brimoor]

Resolves #641

import numpy as np
import fiftyone as fo

polyline = fo.Polyline(points=[])

detection = polyline.to_detection()
assert np.allclose(detection.bounding_box, [0, 0, 0, 0])

detection = polyline.to_detection(mask_size=(2, 2))
assert np.allclose(detection.bounding_box, [0, 0, 0, 0])
assert np.allclose(detection.mask, [[False, False], [False, False]])

[minhtuev]

Nice, thanks Brian!

In [1]: import numpy as np
   ...: import fiftyone as fo
   ...:
   ...: polyline = fo.Polyline(points=[])
   ...:
   ...: detection = polyline.to_detection()
   ...: assert np.allclose(detection.bounding_box, [0, 0, 0, 0])
   ...:
   ...: detection = polyline.to_detection(mask_size=(2, 2))
   ...: assert np.allclose(detection.bounding_box, [0, 0, 0, 0])
   ...: assert np.allclose(detection.mask, [[False, False], [False, False]])

In [2]: