Fix PHP 8.1 "base64_decode(null)" exception calling Conduit's file.upload with no data passed

Summary:
Since PHP 8.1, base64_decode() does not accept passing null as a parameter. Thus first check that data !== null before calling decodeBase64($data) (which then calls PHP's base64_decode()), and throw an exception if it is.

EXCEPTION: (RuntimeException) base64_decode(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=18554ea76ceb), phorge(head=conduitDashboardPanelEdit, ref.master=0d81da590923, ref.conduitDashboardPanelEdit=ab4391b30465)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
  #1 <#2> base64_decode(NULL, boolean) called at [<phorge>/src/applications/files/conduit/FileConduitAPIMethod.php:84]

Closes T15426

Test Plan: Applied this change; afterwards /api/file.upload under "Method Result", "error_info" shows "Unable to decode base64 data!" instead of "base64_decode(): Passing null to parameter #1 ($string) of type string is deprecated" which is more descriptive.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15426

Differential Revision: https://we.phorge.it/D25258