[ENG-1623] Do not return error when pnl/parentSubaccountNumber returns empty array (backport #3291)

[mergify]

Changelist

Fix /v4/pnl/parentSubaccountNumber endpoint to return empty array instead of error when no PNL data matches filters

• Removed the NotFoundError check when pnlData.results.length === 0 - now returns {"pnl":[]} instead of an error
• Added subaccount existence check at the start of the endpoint - returns 404 only if the parent subaccount doesn't exist at all
• This fixes the issue where queries with createdBeforeOrAt filter that excluded all data would return an error instead of an empty array

Test Plan

Added controller tests:

• Get /pnl/parentSubaccountNumber returns empty array when date filter excludes all data - verifies both daily and hourly endpoints return 200 with empty array when createdBeforeOrAt excludes all records
• Get /pnl/parentSubaccountNumber returns empty array when no PNL records exist - verifies empty array is returned when subaccount exists but has no PNL data
• Updated Get /pnl/parentSubaccountNumber with non-existent address returns 404 - verifies 404 is still returned for non-existent addresses

Tested in staging,
before:

after:

Author/Reviewer Checklist

• If this PR has changes that result in a different app state given the same prior state and transaction list, manually add the state-breaking label.
• If the PR has breaking postgres changes to the indexer add the indexer-postgres-breaking label.
• If this PR isn't state-breaking but has changes that modify behavior in PrepareProposal or ProcessProposal, manually add the label proposal-breaking.
• If this PR is one of many that implement a specific feature, manually label them all feature:[feature-name].
• If you wish to for mergify-bot to automatically create a PR to backport your change to a release branch, manually add the label backport/[branch-name].
• Manually add any of the following labels: refactor, chore, bug.

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This is an automatic backport of pull request #3291 done by [Mergify](https://mergify.com).

[linear]

ENG-1623 PnL not running for Telegram dYdX accounts

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