shell: fix SHELL_SUBCMD_COND_ADD with NULL as _handler parameter

In dummy_subcmd_##syntax statement, syntax keyword
should be replaced with _syntax, that is the argument name.

In dummy_##syntax##_handler There are two problems:
 - wrong syntax keyword
 - _handler is an argument

So if NULL is passed as an argument to _handler, and the syntax is abcd,
there is: dummy_##abcd##NULL, what causes a build error.

Signed-off-by: Witold Lukasik <witold.lukasik@nordicsemi.no>