Correct some PhpDoc return types master
Summary: Fix a bunch of PhpDoc return type fixes to make PhpStan complain less. ``` Line src/applications/differential/render/DifferentialChangesetOneUpMailRenderer.php 27 Method DifferentialChangesetOneUpMailRenderer::renderShield() should return string but returns null. ------ Line src/applications/diffusion/request/DiffusionRequest.php 166 Method DiffusionRequest::newFromIdentifier() should return DiffusionRequest but returns null. ------ Line src/applications/files/transform/PhabricatorFileImageTransform.php 219 Method PhabricatorFileImageTransform::newEmptyImage() should return resource but returns GdImage. 366 Method PhabricatorFileImageTransform::getImage() should return resource but returns GdImage. ------ Line src/applications/home/view/PHUIHomeView.php 93 Method PHUIHomeView::buildRevisionPanel() should return PHUIObjectBoxView but returns null. ------ Line src/applications/search/controller/PhabricatorApplicationSearchController.php 980 Method PhabricatorApplicationSearchController::newBuiltinUseActions() should return PhabricatorActionView but returns list. ------ Line src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php 2329 Method PhabricatorApplicationTransactionEditor::newFileTransaction() should return PhabricatorApplicationTransaction but returns null. ``` Test Plan: Read the code. Reviewers: O1 Blessed Committers, mainframe98 Reviewed By: O1 Blessed Committers, mainframe98 Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Differential Revision: https://we.phorge.it/D26691