fix: sass importer can't be undefined (fix: #3390)

[axe-me]

Description

fix: #3390

Only add in the sass importer option when it's defined in the config.
Also checks if the importer config is an array or not;

In this PR, I update some TS types as well.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[patak-dev]

For reference, fixes a regression introduced by #3368 in v2.3.2