updated get_stimuli.py

[najam-tariq]

As discussed in issue #82

[nalmadi]

It looks like the change broke the tests, can you check the test that failed and possibly update it, if needed?
Here's the failed test output:

=========================== short test summary info ============================
FAILED test/parsers/test_AlMadi.py::test_AlMadi - assert 0 == 16

• where 0 = len(array([], dtype=object))
• where array([], dtype=object) = <bound method Series.unique of Series([], Name: trial_id, dtype: object)>()

•  where <bound method Series.unique of Series([], Name: trial_id, dtype: object)> = Series([], Name: trial_id, dtype: object).unique
  

============ 1 failed, 8 passed, 571 warnings in 174.29s (0:02:54) =============
Error: Process completed with exit code 1.