Update ContactsStore: Avoid exceptions on empties

[joshtrichards]

• Resolves: [Bug]: After upgrade to 28.0.1 nonstop errors regarding guestName avatar #42858

Summary

Fixes #42858 and makes things more robust in general in this particular function

TODO

• Confirm backporting (NC28: well, yeah, but not sure about <NC28; if so probably manual required)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[szaimen]

Sorry, not really familiar with that part of the code

[ChristophWurst]

Makes sense

[ChristophWurst]

/backport to stable28

[ChristophWurst]

/backport to stable27

[ChristophWurst]

/backport to stable26

[backportbot-nextcloud]

The backport to stable27 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable27
git pull origin stable27

# Create the new backport branch
git checkout -b fix/foo-stable27

# 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-stable27

Error: Unknown error

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

[backportbot-nextcloud]

The backport to stable26 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable26
git pull origin stable26

# Create the new backport branch
git checkout -b fix/foo-stable26

# 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-stable26

Error: Unknown error

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