add check for root_type specified for json schema generation

[LizardWizzard]

Hi! As recommended here I've added simple check to avoid segfault. In my opinion this solution currently misses user friendly error message, but I dont see any example near this place in codebase for propagating error message. Can you advise me how to properly do this? Also I'll be glad to add test for change I made, so the question is what is the best place for such test? Can I simply add fbs file without root_type and test return code of flatc in generate_code.sh or something else?

Also recommendations for PR says:

For any C++ changes, please make sure to run sh src/clang-format-git.sh

Firstly script is renamed to just clang-format.sh and after I launched it there are format related changes to code that is not related to my changes. So definetly this check should be in CI to protect the health of the codebase.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[LizardWizzard]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[aardappel]

You don't have the right clang-format script because you haven't rebased to latest master, but that's ok in this case.

Thanks!