R1:2ffbef882038

Fix PHP 8.1 "trim(null)" exception which blocks rendering Reports' Burnup Rate page

Summary:
Since PHP 8.1, passing a null string to `trim()` is deprecated.

Thus first check that $row['oldValue'] is not null before trimming it.

Closes T15392

Test Plan: Applied this change; afterwards "Burnup Rate" page at `/maniphest/report/burn/` got correctly rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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