[Fix] Fix features variable may be None in points sampler (#2234)

mmcv/ops/points_sampler.py

@@ -102,7 +102,7 @@ class PointsSampler(nn.Module):
         """
         if points_xyz.dtype == torch.half:
             points_xyz = points_xyz.to(torch.float32)
-        if features.dtype == torch.half:
+        if features is not None and features.dtype == torch.half:
             features = features.to(torch.float32)
 
         indices = []