Fix DIET Classifier crash on empty input

[enochlev]

Simple Fix to prevent DIET classifier from crashing on empty string:

• From what I understand all other components don't crash when input string is empty except for the DIET classifier component
• Its important because when using an API to access RASA, we don't want the program to crash if the input is empty.
• I can provide more details of where it crashes, but it should be very simple to find

Status (please check what you already did):

• [ ] added some tests for the functionality
• [ ] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)