enter typing animation doesn't work with prompt that return value

[jcubic]

Issue summary

When there is prompt that return string, exec and enter typing animation doesn't work

Expected behavior

Animation works as in other places

Actual behavior

Animation doesn't work

Steps to reproduce

1. Open Fake Linux Terminal Demo
2. Run $.terminal.active().exec(['ls'], { typing: true });
3. Nothing happens, because the value is ignored and callback function (for prompt) is not called.