Learning header Help link renders href="null" when SUPPORT_URL is unset #685

The learning header's Help link renders as <a class="text-gray-700" href="null"> when the MFE config has no SUPPORT_URL. That's the default: neither the platform nor Tutor sets it for the learning MFE. Clicking it navigates to a relative null path.

Cause: src/learning-header/LearningHeaderHelpLink.jsx builds the href with a template literal:

<a className="text-gray-700" href={`${getConfig().SUPPORT_URL}`}>{intl.formatMessage(messages.help)}</a>

so an unset SUPPORT_URL (null) becomes the string "null", and the link always renders.

Expected: don't render the Help link when SUPPORT_URL is empty, as the frontend-base shell's Help widget does. #276 (2022, closed unmerged) proposed that.

Environment: Tutor main and verawood, learning MFE course home and unit pages, 2026-09-23; component unchanged on master since #545 (2024-10). This is the "Help link does not redirect" half of openedx/wg-build-test-release#579 (TC-00021). Found by the Open edX end-to-end test suite.