nixos/openldap: fix assertion

In commit d43dc68d, @Mic92 split the
rootpw option to allow specifying it in a file kept outside the Nix
store, as an alternative to specifying the password directly in the
config.

Prior to that, rootpw's type was `str`, but in order to allow both
alternatives, it had to become `nullOr str` with a default of `null`. So
I can see why this assertion, that either rootpw or rootpwFile are
specified, makes sense to add here.

However, these options aren't used if the configDir option is set, so as
written this assertion breaks valid configurations, including the
configuration used by nixos/tests/ldap.nix.

So this patch fixes the assertion so that it doesn't fire if configDir
is set.