sequenceDiagram: dotted line for alt and empty bracket should be hidden

[brookhong]

This is the best javascript implementation to render sequence diagram from text I have ever found. Thanks for your great works.

I found there are some small issues of alt.

1. The line in middle of alt box is solid, which is a little strange, as most output of other tools are using dotted line.
2. For the else branch of alt block, if we don't append description after else, an empty bracket is displayed, which is also not necessary. And in most cases, for else branch, we actually don't append any description. So some empty brackets are displayed, that is not reasonable.