Ignore blank lines in *.txt labels (ultralytics#3366)

Fix for ultralytics#958 (comment)

(cherry picked from commit 4d4a2b0)

utils/datasets.py

@@ -474,7 +474,7 @@ def cache_labels(self, path=Path('./labels.cache'), prefix=''):
                 if os.path.isfile(lb_file):
                     nf += 1  # label found
                     with open(lb_file, 'r') as f:
-                        l = [x.split() for x in f.read().strip().splitlines()]
+                        l = [x.split() for x in f.read().strip().splitlines() if len(x)]
                         if any([len(x) > 8 for x in l]):  # is segment
                             classes = np.array([x[0] for x in l], dtype=np.float32)
                             segments = [np.array(x[1:], dtype=np.float32).reshape(-1, 2) for x in l]  # (cls, xy1...)