cstyle: understand macro params can be empty

It's not uncommon to have empty parameters in code generator macros,
usually when multiple parameters are concatenated or stringified into a
single token or literal. So, exclude the space-before-comma check, which
will allow construction like MACRO_CALL(foo, , baz).

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16840