Since PHP 8.1, `base64_decode()` does not accept passing null as a parameter. Thus first check via `phutil_nonempty_string()` before passing data.
Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.
```
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