project-card.cjsx: provide default value for props.project

[hrdl-github]

Fixes #6855 .

Provide default value for props.project. I haven't followed the testing protocol as I'm not using a staging environment.

Required Manual Testing

• Does the non-logged in home page render correctly?
• Does the logged in home page render correctly?
• Does the projects page render correctly?
• Can you load project home pages?
• Can you load the classification page?
• Can you submit a classification?
• Does talk load correctly?
• Can you post a talk comment?

Review Checklist

• Does it work in all major browsers: Firefox, Chrome, Edge, Safari?
• Does it work on mobile?
• Can you npm ci and app works as expected?
• If the component is in coffeescript, is it converted to ES6? Is it free of eslint errors? Is the conversion its own commit?
• Are the tests passing locally and on GitHub Actions?

Optional

• Have you replaced any ChangeListener or PromiseRenderer components with code that updates component state?
• If changes are made to the classifier, does the dev classifier still work?
• Have you resized and compressed any image you've added?
• Have you added in flow type annotations?
• Have you followed the Springer guidelines for commit messages?

[coveralls]

coverage: 57.192%. remained the same when pulling f8b12f1 on hrdl-github:fix-project-props into a4996fb on zooniverse:master.

[eatyourgreens]

Looks good! Thanks for this.