`error-message` believes that a joined array is empty when elements are conditionally pushed

[Jake Bailey]

Code like the below appears in TypeScript's test runner; an array is used to collect error messages, which are joined and thrown when non-empty. But, error-message thinks that this error list could be empty (though the length is checked) and errors.

const errs = [];

if (someCondition) {
    errs.push("hello");
}

if (errs.length) {
    throw new Error(errs.join("\n     "));
}

(Personally, I think that this check is a little too strong; it's not clear that a linter can really "prove" that an array is or isn't empty, so using that info in the lint feels iffy. But, there's a length check, which is theoretically enough for this case.)