fix(build): correct get_default_password definition for sub-project inclusion

Description

Add a tighter security in PasswordKeeper around the get_default_password method to make sure that there is no build error when including this header in a project where the SIGHT_DEFAULT_PASSWORD is not defined

Closes #820 (closed)

How to test it?

Add #include <sight/ui/base/Preferences.hpp> header in a sight-project specific header.

Try to build