cgen: avoid generation of empty or block

[StunxFS]

With this PR, code like the following:

module main

fn res() ! {}

fn main() {
	res() or {}
}

It will be generated in C like this:

VV_LOCAL_SYMBOL void main__main(void) {
        _result_void _t1 = main__res();
        (void)_t1;
 ;
}

Previously it was generated like this:

VV_LOCAL_SYMBOL void main__main(void) {
        _result_void _t1 = main__res();
        if (_t1.is_error) {
                IError err = _t1.err;
        }

 ;
}

_{Huly®: V_0.6-21221}

[spytheman]

Excellent work.

Can you please also add a small test pair of .vv/.c.must_have files in vlib/v/gen/c/testdata/ ?

[felipensp]

Nice. Good find.

[StunxFS]

Excellent work.

Can you please also add a small test pair of .vv/.c.must_have files in vlib/v/gen/c/testdata/ ?

@spytheman added test 👍🏽