Update news_preprocess.py

[Sanky18]

Instead of using np.concatenate, try using a list comprehension to flatten your nested list. Ensure that none of the sublists in your words list are empty. An empty sublist can also cause issues when creating a NumPy array.

[chuchunchi]

Hi @Sanky18,
I try to re-run this repo but get the error and fixed it the same way as your PR, however, please let me know if you've also encounter the following error when running train.py after preprocess the news.tsv with such method. Thanks.

Traceback (most recent call last):
  File "/Users/chuchun/Documents/NRMS_new-for-MIND/src/main.py", line 228, in <module>
    training_data = np.array(training_data)
                    ^^^^^^^^^^^^^^^^^^^^^^^
ValueError: setting an array element with a sequence. The requested array has an inhomogeneous shape after 2 dimensions. The detected shape was (375398, 4) + inhomogeneous part.