Check free space only if source exists and is dir

[solracsf]

Summary

As disk_free_space() expects a directory, let's check if it is one before runing the code, return FileInfo::SPACE_UNKNOWN otherwise.

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]

LGTM now but didnt test

[solracsf]

/backport to stable26

[solracsf]

/backport to stable25

[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

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

[backportbot-nextcloud]

The backport to stable25 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable25
git pull origin/stable25

# Create the new backport branch
git checkout -b fix/foo-stable25

# 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-stable25

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

[solracsf]

/backport to stable26

[solracsf]

/backport to stable25