fix: null check on hasInstance

[jimmywarting]

The purpose of this PR is:

... to not throw if comparing null is a instance of FormData

This is what had to change:

... first check if obj is truthy

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• [x] I prefixed the PR-title with docs:, fix(area):, feat(area): or breaking(area):
• [x] I Added unit test(s)

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• fix https://github.com/jimmywarting/FormData/pull/136#pullrequestreview-767786804

[github-actions[bot]]

:tada: This PR is included in version 4.0.10 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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