Update instructions in assignment 6B2.2 to exclude other inputs

[Enturk]

A diligent student was unit testing empty strings and the feedback for 6B2.2 wasn't properly accounting for empty strings in the input list. Here's their submission:

https://blockpy.cis.udel.edu/blockpy/view_submissions/4718/1893373/172?embed=True#6b2-2-existing-functions