no task in certain bmi3d experiments

laser only task doesn't have a 'task' table because there is nothing to record (cursor, e.g.)
but this table is currently required by proc_exp

```
Traceback (most recent call last):
File "", line 19, in
aopy.preproc.proc_exp(data_dir, files, subj_dir, result_filename, overwrite=True)
File "/home/aolab/leo/analyze/aopy/preproc.py", line 1104, in proc_exp
bmi3d_data, bmi3d_metadata = parse_bmi3d(data_dir, files)
File "/home/aolab/leo/analyze/aopy/preproc.py", line 634, in parse_bmi3d
data, metadata = _parse_bmi3d_v0(data_dir, files)
File "/home/aolab/leo/analyze/aopy/preproc.py", line 667, in _parse_bmi3d_v0
bmi3d_task, bmi3d_task_metadata = load_bmi3d_hdf_table(data_dir, bmi3d_hdf_filename, 'task')
File "/home/aolab/leo/analyze/aopy/data.py", line 562, in load_bmi3d_hdf_table
raise ValueError(f"{table_name} not found in {filename}")
ValueError: task not found in /data/raw/hdf/test20210608_11_te1796.hdf
```