R1:747d7db24161

Fix a PHP 8.1/8.2 deprecated use of ltrim and rtrim with a NULL argument

Summary:
These calls were preventing notification servers configuration to be properly initialized.
Indeed, PHP 8.X is stricter concerning
This behavior is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Fix T15598

Test Plan:
Sign in as an administrator, configure the notification server without filling admin path field,
you shouldn't get both an RuntimeException and a warning indicating that…