[BUGFIX] Prevent undefined array key warning

[peterkraume]

Fixes: #565

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

[RinyVT]

Thanks for the PR! I've changed the code a bit by using !isset and ?? to make it a bit more readable and we do not need the null checks that array_key_exists provides, do you agree?

[peterkraume]

I'm perfectly fine with your changes. Thx for taking care!