Fix false positives for functions inside of math functions in length-zero-no-unit

[tomyo]

Clearly describe the bug
The rule length-zero-no-unit is applying fixes inside level 4 math functions.

This test exercises exactly what it shouldn't happen:

https://github.com/tomyo/stylelint/blob/eb419ef754ac442f9f3b9f7164fccb938b99adb3/lib/rules/length-zero-no-unit/__tests__/index.js#L332

Why?
It converts this:

padding: calc(var(--foo, 0px) + 10px);

Into this:

padding: calc(var(--foo, 0) + 10px);

If --foo is not defined, the fallback value 0 is used, resulting in calc(0 + 10px); which is invalid.

What stylelint configuration is needed to reproduce the bug?

{
  "rules": {
    "length-zero-no-unit": true
  }
}

Which version of stylelint are you using?

13.12.0

What did you expect to happen?

No unit strips should happen inside level 4 math functions, just for safety.

In the link to the test I gave, something like this is what I would expect instead:

{
			description: 'ignore calc, and inner functions',
			code: 'padding: calc(var(--foo, 0in) + 10px) 0px;',
			fixed: 'padding: calc(var(--foo, 0in) + 10px) 0;',
}

[jeddy3]

@tomyo Thanks for the report and clear write-up of the problem.

No unit strips should happen inside level 4 math functions, just for safety.

Yes, that sounds appropriate.

Now that your related pull request (#5203) is merged, I'll label this is ready to implement.