fix: Check strict cookies for image proxy

[ChristophWurst]

#2153 little sibling. It makes sense to check for the cookie existence here too.

How to test

1. Set up an account
2. Open a HTML message with external images
3. Click Show images and Show images temporarily

main: images show
here: images show

[ChristophWurst]

/backport to stable2.2

[ChristophWurst]

/backport to 1.15

[GretaD]

didnt test

[hamza221]

Tested

[ChristophWurst]

/backport to stable3.2

[backportbot-nextcloud]

The backport to 1.15 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout 1.15
git pull origin 1.15

# Create the new backport branch
git checkout -b fix/foo-1.15

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-1.15

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport