LoadCredentials: do not assert on invalid syntax

LoadCredentials: do not assert on invalid syntax

LoadCredentials=foo causes an assertion to be triggered, as we
are not checking that the rvalue's right hand side part is non-empty
before using it in unit_full_printf.

Fixes #19178

# printf [Service]nLoadCredential=passwd.hashed-password.rootn > hello.service
# systemd-analyze verify ./hello.service
...
Assertion 'format' failed at src/core/unit-printf.c:232, function unit_full_printf(). Aborting.
Aborted (core dumped)