Correct some PhpDoc return types
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