Do not set default $g_path from CLI

The set_default_path() function relies on $_SERVER['SCRIPT_NAME'], which
does not contain an URL when running from CLI.

This results in a most likely invalid URL being stored in $g_path, which
could lead to unexpected behavior or errors later on.

Simply refuse to set $g_path in this case, and print a warning on
screen. Note that this should normally not happen, except in development
environments.

Fixes 0037313