[Examples] Check key exists in datasets first

[oToToT]

What does this PR do?

Correctly check the key exists before accessing it in some example scripts. I guess this is probably a mistake when writing example scripts.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

No tests since I didn't see any tests related to examples. Maybe someone could point it out for me.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

Probably @bhadreshpsavani would like to review this according to the log from git blame.
Or @sgugger, @patil-suraj as this is about examples.

[bhadreshpsavani]

Hi @oToToT,
Thanks for the PR, The changes seem valid to me. It should be as per changes in this PR.
@sgugger what's your view on these changes?

[sgugger]

Thanks for fixing, it's better this way indeed!

[oToToT]

Hi, @sgugger
I'm not familiar with how huggingface do with pull requests, did I miss something to make it be merged?
Or I just need to stay and wait it.

Thanks!

[sgugger]

Thanks for the ping! I just forgot to click the merge button 🤦