Return stderr if stdout is empty

This also includes a note if both are empty.  Both having content is not
yet handled.

zig/sdl/hello_sdl.zig

        std.debug.print("'{s}' ", .{arg});
    }
    const result = try std.ChildProcess.exec(.{ .allocator = allocator, .argv = argv, .max_output_bytes = 1024 * 1024 });
    std.debug.print("stdout: '{s}'\n", .{result.stdout[0..std.math.min(100, result.stdout.len)]});
    std.debug.print("stderr: '{s}'\n", .{result.stderr});
    defer {
        // FIXME: allocator.free(result.stdout);
    if (result.stdout.len > 0) {
        allocator.free(result.stderr);
        return result.stdout;
    } else if (result.stderr.len > 0) {
        allocator.free(result.stdout);
        return result.stderr;
    } else {
        allocator.free(result.stdout);
        allocator.free(result.stderr);
        return std.fmt.allocPrint(allocator, "<no output>", .{});
    }
    return result.stdout;
}
// tests