Do not throw error on review comments

[hyperupcall]

When adding comments under a review, the action would previously throw an error due to a null body.

Tried looking for the body, but as mentioned in #35, the body of pull request review comments doesn't seem to be documented. The event payload seems to have a review property, but it seems to be associated with the overall review rather than a particular review comment.

review: {
    _links: { html: [Object], pull_request: [Object] },
    author_association: 'MEMBER',
    body: null,
    commit_id: '58e4709c661af8de19e52af500d6808ff0a0504f',
    html_url: 'https://github.com/SchemaStore/schemastore/pull/4587#pullrequestreview-2705553345',
    id: 2705553345,
    node_id: 'PRR_kwDOAZi2O86hQ3fB',
    pull_request_url: 'https://api.github.com/repos/SchemaStore/schemastore/pulls/4587',
    state: 'commented',
    submitted_at: '2025-03-21T11:09:40Z',
    user: {

If one truly needed this feature, somebody could iterate through pulls.listReviewComments for the current PR, and compare it's update_at property with the current review comment's submitted_at property. Although, of course it would lead to bad results if two people comment within the same second. Property values may also be unexpected when updating comments.

For now, simply set body so that no crashes occur.

[orta]

Yeah, this looks good to me 👍🏻