Add test for empty predictions

Reproduces issue: cheind#49

motmetrics/tests/test_metrics.py

@@ -174,3 +174,45 @@ def compute_motchallenge(dname):
         [0.624296, 0.799176, 0.512211, 0.602640, 0.940268, 18.0, 6, 10, 2, 58, 602, 14, 13, 0.555116, 0.330177],
     ])
     np.testing.assert_allclose(summary, expected, atol=1e-3)
+
+
+def test_optimal_assignment_metrics_with_empty_groundtruth():
+    acc = mm.MOTAccumulator(auto_id=True)
+    # Empty groundtruth.
+    acc.update([], [1, 2, 3, 4], [])
+    acc.update([], [1, 2, 3, 4], [])
+    acc.update([], [1, 2, 3, 4], [])
+    acc.update([], [1, 2, 3, 4], [])
+
+    mh = mm.metrics.create()
+    metr = mh.compute(acc, return_dataframe=False, metrics=[
+        'num_matches', 'num_false_positives', 'num_misses',
+        'idtp', 'idfp', 'idfn',
+    ])
+    assert metr['num_matches'] == 0
+    assert metr['num_false_positives'] == 16
+    assert metr['num_misses'] == 0
+    assert metr['idtp'] == 0
+    assert metr['idfp'] == 16
+    assert metr['idfn'] == 0
+
+
+def test_optimal_assignment_metrics_with_empty_predictions():
+    acc = mm.MOTAccumulator(auto_id=True)
+    # Empty predictions.
+    acc.update([1, 2, 3, 4], [], [])
+    acc.update([1, 2, 3, 4], [], [])
+    acc.update([1, 2, 3, 4], [], [])
+    acc.update([1, 2, 3, 4], [], [])
+
+    mh = mm.metrics.create()
+    metr = mh.compute(acc, return_dataframe=False, metrics=[
+        'num_matches', 'num_false_positives', 'num_misses',
+        'idtp', 'idfp', 'idfn',
+    ])
+    assert metr['num_matches'] == 0
+    assert metr['num_false_positives'] == 0
+    assert metr['num_misses'] == 16
+    assert metr['idtp'] == 0
+    assert metr['idfp'] == 0
+    assert metr['idfn'] == 16