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

src/applications/files/conduit/FileUploadConduitAPIMethod.php

···
     $view_policy = $request->getValue('viewPolicy');
 
     $data = $request->getValue('data_base64');
+    if (!phutil_nonempty_string($data)) {
+      throw new Exception(pht('Field "data_base64" must be non-empty.'));
+    }
     $data = $this->decodeBase64($data);
-
     $params = array(
       'authorPHID' => $viewer->getPHID(),
       'canCDN' => $can_cdn,