Fix isNil import

[jakubjanczyk]

Problem/Feature

Previous import if isNil function from lodash.isnil was incorrect - for the specific lodash package the function needs to be imported with default import

Guidelines

Make sure the pull request:

• [ ] Follows the established folder/file structure
• [ ] Adds unit tests
• [ ] If it is a refactor or change to an existing component, have you verified it won't break existing Cypress tests or have you updated them?
• [ ] Did you verify some accessibility (a11y) basics?
• [ ] Adds/updates stories. Guidelines
• [ ] Adds/updates documentation (ie proptypes) Guidelines
• [ ] Has it been tested in Help Scout's supported browsers?
• [ ] Requests review from designer of the feature
• [ ] Add label (bug? feature?)

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[tinkertrain]

Fix incorporated into https://github.com/helpscout/hsds-react/pull/1036