execute_func: Newline only for non-empty returns

[wallento]

For void functions, the return value array is empty. Don't print the newline then.

[yamt]

doesn't this confuse programs parsing the output, like our test scripts?

[wallento]

I am happy to adopt the test scripts then, but so far I am not sure if thats the reason for the 9 failing checks.
Those are just empty lines without useful information, there must be a better way to test if needed. Overall, I think this feature is not really useful for embedding wamr, as it always prints this information. I am also happy to adopt it to only print given certain defines.

[wallento]

The failing checks all seem timeouts, so I think no test checks for this.