fix(imageview): Don't try to load unresolved images

[nickvergessen]

📝 Summary

Currently a lot of people on our instance get bruteforce protected in Talk regularly. After some log reading it seems to always happen when interactive widgets are involved.
After some trying the smallest possible test is:

1. Create a test.md file with the following content: (on our instance it's .attachment links with wrong file ids)

   ![](C.png)

2. Copy the /f/... link and post it into a chat
3. Check the browser console for calls to index.php/call/undefined

After some debugging I found this trace. The problem is the "default" resolver responds:

text/src/components/Editor.provider.js

Lines 91 to 94 in 648cf20

	resolve(src) {
	logger.warn('No attachment resolver provided. Some attachment sources cannot be resolved.')
	return [src]
	},

This means that

text/src/nodes/ImageView.vue

Line 245 in 99e7a7a

this.attachment = await this.$attachmentResolver.resolve(this.src)

makes this.attachment = [this.src]
in the Promise it sets img.src = this.attachment.previewUrl but previewUrl is undefined as only 0 exists.
This will make the browser load undefined relative to the current page which results in https://localhost/index.php/call/undefined
This however looks like you are trying to open a conversation with the token undefined, but you have no access to it => bruteforce registered.

by simply checking if previewUrl exists and otherwise jumping into the fail mode, it prevents the request to load index.php/call/undefined and so also no bruteforce attempt it registered.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

[Antreesy]

Should be enough for a fix. Waiting for maintainers input

[max-nextcloud]

Thanks for looking into this. Code change looks good and matches the description. Let's get this in.

[nickvergessen]

/backport to stable29