fix(storage): skip docdb watcher reconfiguration on empty file read

[infernus01]

Changes

Fix flaky TestWatchBackend that fails intermittently in the release pipeline but passes locally.

Problem

os.WriteFile is not atomic — it truncates the file (O_TRUNC) writing new content. When the fsnotify watcher goroutine reads the file during this truncation window, it gets an empty string. This causes:

1. MONGO_SERVER_URL env var set to ""
2. docstore.OpenCollection fails: "MONGO_SERVER_URL environment
   variable is not set"
3. Watcher sends nil to the unbuffered backendChan and blocks
4. Subsequent fsnotify events can never be processed — deadlock

The race is timing-dependent: on a fast local machine the watcher processes events before the next file write starts; on a busy CI node the truncation window is wide enough to hit.

Fix

Skip reconfiguration when the file read returns an empty value. An empty MONGO_SERVER_URL is never a valid configuration — reconfiguring with it always fails. In production, Kubernetes secret mounts use atomic symlink swaps (..data), so transient empty reads don't occur; this guard makes the watcher robust to non-atomic writes without changing any valid production behavior.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

/kind bug

[jkhelil]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jkhelil

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [jkhelil]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[jkhelil]

/lgtm

[jkhelil]

/cherry-pick release-v0.28.x

[tekton-robot]

✅ Cherry-pick to release-v0.28.x successful!

A new pull request has been created to cherry-pick this change to release-v0.28.x.

PR: #1770

Please review and merge the cherry-pick PR.