Add linter rule for not using falsiness of types

In several places in our code, we have run into bugs where we rely on the falsy-ness of a type or accidentally rely on it. We should add a linter rule to make checks explicit and never do things like if {} by accident.

The issue does not name any files, tests, linter, or entry point to inspect. First identify the project's linting setup and examples of accidental type falsiness; done should mean an explicit linter rule catches the described checks without unwanted violations.

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