fix(lib/core/utils/aggregateChecks.js): don’t trust indexOf(undefined) to do the right thing.

[zkf]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message(s) follow our guidelines: https://github.com/dequelabs/axe-core/blob/develop/doc/code-submission-guidelines.md#git-commits
• Changes to rules and checks appropriately support Shadow DOM
• Changes have been tested in major browsers and Assistive Technologies
• Tests for the changes have been added (for bug fixes / features)
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Description of the changes

• Github issue: Off-by-one error and unchecked parameter in aggregateChecks.js #960

Does this PR introduce a breaking change?

• Yes
• No

Other information

[WilcoFiers]

Awesome, thanks for the contribution.