fix: add optional chaining before access to the targetRevision field

[daengdaengLee]

Hello,

This PR is related to issue #22545.
For the /applications page, we were missing some null/undefined checks, so I've added optional chaining.
I couldn't reproduce this issue, so I'm not entirely sure if this fix will solve it definitively.
However, I identified and corrected the suspicious code in the UI based on the error details from the issue.

Here's the detailed flow:

• In ui/src/app/app.tsx, the applications.component is registered to the /applications path within routes.
• The applications.component is actually the ApplicationsContainer component, declared in ui/src/app/applications/components/applications-container.tsx.
• The component registered for the root path (without sub-paths) is ApplicationsList (ui/src/app/applications/components/applications-list/applications-list.tsx).
• The ApplicationsList component uses ApplicationsTable (ui/src/app/applications/components/applications-list/applications-table.tsx).
• The ApplicationsTable component uses ApplicationsLabels (ui/src/app/applications/components/applications-list/applications-labels.tsx).
• Inside ApplicationsLabels, the code getAppDefaultSource(app).targetRevision is used.
• getAppDefaultSource (ui/src/app/applications/components/utils.tsx) can return null or undefined.
• I added optional chaining before trying to read targetRevision from the return value: getAppDefaultSource(app)?.targetRevision.

Checklist:

• [b] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[ishitasequeira]

Thanks @daengdaengLee for the PR. LGTM