do_action should not pass empty string by default

[nacin]

function test( $a = true ) {
   var_dump( $a );
}
add_action( 'test', 'test' );
add_action( 'test', 'test', 10, 0 );
do_action( 'test' );

First example is bool(true), but the second is a zero-length string.

In this case, since do_action() is not passing any arguments, then no arguments should be passed to test() -- even if by default, one argument gets passed.

The issue here is how do_action() is defined: function do_action( $tag, $arg = '' );. There's our zero-length string. Of note, apply_filters() does not have this same issue.

The solution looks something like what I have attached. Has yet to be benchmarked.

Of note, this does not appear to have any direct implications for passing all args by default, as proposed in #14671, but I wanted to reference it anyway.