rP976a21f6584b

AphrontFormControl: fix regression for some specific Captions

Summary:
Fix a specific problem when visiting some specific pages like this one:

/auth/config/edit/?provider=PhabricatorPhabricatorAuthProvider:

Regression introduced in:

562d36ef5f9e83dba5e3ca0244f9c4af9e1f7b2b

Stack trace:

[Fri May 19 14:23:35.506028 2023] [php7:notice] [pid 20439] [client 127.0.0.1:39692] [2023-05-19 14:23:35] EXCEPTION: (InvalidArgumentException) Call to phutil_nonempty_string() expected null or a string, got…