Allow blog posts without cover image

[zohaad]

I'm getting the following error in my deploy GitHub action, and I think it's because I don't have a cover image:

2022-04-30T21:18:44.3758138Z yarn install v1.22.18
2022-04-30T21:18:44.4447506Z [1/4] Resolving packages...
2022-04-30T21:18:44.6143394Z success Already up-to-date.
2022-04-30T21:18:44.6231339Z Done in 0.25s.
2022-04-30T21:18:44.7779136Z yarn run v1.22.18
2022-04-30T21:18:44.8091286Z $ APP_ENV=prod yarn build && next export
2022-04-30T21:18:44.9928841Z $ next build && yarn build:rss
2022-04-30T21:18:45.4634460Z info  - Loaded env from /home/runner/work/blog.***/blog.***/.env
2022-04-30T21:18:45.6560265Z info  - Checking validity of types...
2022-04-30T21:18:48.5089458Z warn  - No ESLint configuration detected. Run next lint to begin setup
2022-04-30T21:18:48.5106738Z info  - Creating an optimized production build...
2022-04-30T21:18:53.7098316Z info  - Compiled successfully
2022-04-30T21:18:53.7099406Z info  - Collecting page data...
2022-04-30T21:18:54.4366556Z info  - Generating static pages (0/6)
2022-04-30T21:18:54.4583991Z info  - Generating static pages (1/6)
2022-04-30T21:18:54.4615839Z cover missing for post: {
2022-04-30T21:18:54.4616625Z   id: 'b93ca130-a39e-489d-b6d6-ccf914e96eec',
2022-04-30T21:18:54.4617233Z   slug: 'fun-quotes',
2022-04-30T21:18:54.4617750Z   author: 'Zohaad Fazal',
2022-04-30T21:18:54.4618436Z   date: { start: '2022-04-28', end: null, time_zone: null },
2022-04-30T21:18:54.4618985Z   published: true,
2022-04-30T21:18:54.4619484Z   title: 'Fun quotes',
2022-04-30T21:18:54.4620093Z   subtitle: 'Some cool quotes for me to copy-paste',
2022-04-30T21:18:54.4620665Z   language: 'English'
2022-04-30T21:18:54.4621104Z }
2022-04-30T21:18:54.5004186Z info  - Generating static pages (2/6)
2022-04-30T21:18:54.6339541Z 
2022-04-30T21:18:54.6340727Z Error occurred prerendering page "/fun-quotes". Read more: https://nextjs.org/docs/messages/prerender-error
2022-04-30T21:18:54.6341747Z TypeError: Cannot read properties of undefined (reading 'file')
2022-04-30T21:18:54.6342666Z     at Slug (/home/runner/work/blog.***/blog.***/.next/server/pages/[slug].js:572:192)
2022-04-30T21:18:54.6343811Z     at d (/home/runner/work/blog.***/blog.***/node_modules/react-dom/cjs/react-dom-server.node.production.min.js:33:498)
2022-04-30T21:18:54.6345023Z     at bb (/home/runner/work/blog.***/blog.***/node_modules/react-dom/cjs/react-dom-server.node.production.min.js:36:16)
2022-04-30T21:18:54.6346253Z     at a.b.render (/home/runner/work/blog.***/blog.***/node_modules/react-dom/cjs/react-dom-server.node.production.min.js:42:43)
2022-04-30T21:18:54.6347444Z     at a.b.read (/home/runner/work/blog.***/blog.***/node_modules/react-dom/cjs/react-dom-server.node.production.min.js:41:83)
2022-04-30T21:18:54.6348907Z     at Object.exports.renderToString (/home/runner/work/blog.***/blog.***/node_modules/react-dom/cjs/react-dom-server.node.production.min.js:52:138)
2022-04-30T21:18:54.6349993Z     at Object.renderPage (/home/runner/work/blog.***/blog.***/node_modules/next/dist/server/render.js:621:45)
2022-04-30T21:18:54.6350995Z     at Object.defaultGetInitialProps (/home/runner/work/blog.***/blog.***/node_modules/next/dist/server/render.js:301:51)
2022-04-30T21:18:54.6351948Z     at Function.getInitialProps (/home/runner/work/blog.***/blog.***/.next/server/chunks/859.js:497:20)
2022-04-30T21:18:54.6353217Z     at Object.loadGetInitialProps (/home/runner/work/blog.***/blog.***/node_modules/next/dist/shared/lib/utils.js:69:29)
2022-04-30T21:18:54.6354615Z info  - Generating static pages (4/6)
2022-04-30T21:18:55.5316186Z info  - Generating static pages (6/6)
2022-04-30T21:18:55.5316748Z 
2022-04-30T21:18:55.5317040Z > Build error occurred
2022-04-30T21:18:55.5317675Z Error: Export encountered errors on following paths:
2022-04-30T21:18:55.5318471Z 	/fun-quotes
2022-04-30T21:18:55.5319306Z     at /home/runner/work/blog.***/blog.***/node_modules/next/dist/export/index.js:493:19
2022-04-30T21:18:55.5319983Z     at runMicrotasks (<anonymous>)
2022-04-30T21:18:55.5320643Z     at processTicksAndRejections (node:internal/process/task_queues:96:5)
2022-04-30T21:18:55.5321570Z     at async Span.traceAsyncFn (/home/runner/work/blog.***/blog.***/node_modules/next/dist/trace/trace.js:74:20)
2022-04-30T21:18:55.5322512Z     at async /home/runner/work/blog.***/blog.***/node_modules/next/dist/build/index.js:956:17
2022-04-30T21:18:55.5323439Z     at async Span.traceAsyncFn (/home/runner/work/blog.***/blog.***/node_modules/next/dist/trace/trace.js:74:20)
2022-04-30T21:18:55.5324370Z     at async /home/runner/work/blog.***/blog.***/node_modules/next/dist/build/index.js:830:13
2022-04-30T21:18:55.5325305Z     at async Span.traceAsyncFn (/home/runner/work/blog.***/blog.***/node_modules/next/dist/trace/trace.js:74:20)
2022-04-30T21:18:55.5326302Z     at async Object.build [as default] (/home/runner/work/blog.***/blog.***/node_modules/next/dist/build/index.js:80:25)
2022-04-30T21:18:55.5600232Z error Command failed with exit code 1.```