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)

[CLAassistant]

All committers have signed the CLA.

[znat]

Hey @enochlev , thanks for your PR. Can you please elaborate on the bug you are fixing? I tried to reproduced the following:

• rasa run --enable-api (with a trained model)
• POST the following to /model/parse, which returns an regular inference payload

{
  "text": ""
}

Anything I didn't understand correctly?

[m-vdb]

Hi there! Thanks for your contribution. As the PR has been stale for a while, and without a clear understanding of which bug the PR is attempting to fix, I'm closing it. If you're still impacted by the issue, please open a new one with a description of the issue you're experiencing (logs + commands you've been running). Cheers!