fix: bypass descendant combinator typeerror

[computnik]

Motivation and context:

The nwsapi parsing gives TypeError in cases like below:

  &:disabled {
      svg path {
          fill: var(--color-brand-primary)
      } 
  }

TypeError: Cannot read properties of null (reading 'parentElement')

This change checks for null value when parsing the Descendant combinator, and can potentially bypass TypeErrors like in #75

[ahayes91]

Thank you!

[dperini]

@computnik
I changed strategy for resolving combinators for when there are extreme combinators nesting.
At each nesting level a new variable is created/retained so it will be impossible to overwrite them.
See this commit 12f9f59