fix: only check for source position when --show-params is set

[gdsoumya]

Fixes https://github.com/argoproj/argo-cd/issues/20670

This PR ensures that app get works for multi source apps even when --source-position isn't provided. Source position check is only necessary if the user also sets --show-params

Checklist:

• [ ] 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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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 55.18%. Comparing base (35b9158) to head (4f311e8). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/argocd/commands/app.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #20682 +/- ## ========================================== - Coverage 55.20% 55.18% -0.02% ========================================== Files 324 324 Lines 55256 55257 +1 ========================================== - Hits 30505 30496 -9 + Misses 22143 22140 -3 - Partials 2608 2621 +13 ```

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[gdsoumya]

/cherry-pick release-2.13