Bug 433686: patch for assert crash in animation curve docker

As stated in the bug 433686, Krita would crash giving an assert error where it was trying to access an element of an empty list. I've just added an if statement to check if it is empty and if yes, return.

I have ran the relevant unit tests, and they passed. However, as I'm not entirely sure about how the feature works, please verify if it causes a problem.

Test Plan

(as mentioned in the bug description)

1. Open a new document,
2. Then open the animation workspace.
3. Then go to the animation curves docker
4. Right click just below the name (background layer) in the empty area

Earlier, Krita would crash. Now it does not crash and doesn't give the message ASSERT: "!isEmpty()"

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.