R1:1d8483a9bd85

Fix PHP 8.1 "strlen(null)" exception which blocks rendering the "Manage Menu" page

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.
However, the parameter `$item_id` is a numeric value when populated.
Thus replace `strlen()` with `phutil_nonempty_scalar()` as `phutil_nonempty_string()` could break a Conduit API consumer sending a numeric value.

Note: this may…