I hit this question since I'm wrestling with the same problem. This is what I've figured out.
Protractor (and Karma for the jsunit testing of Angular) requires node, but this isn't a large obstacle since you can pull in and install node using Maven, and use it to pull in yarn to get angular setup and built at deploy and test time. It is also possible to set up a proxy server from the node server to the Spring Boot App so that the REST endpoints will work.
What I want (and what I'm guessing the original poster wants) is to figure out a way to start up protractor without starting the node server. I don't want that server started because as part of the end to end testing I want to verify the spring app is serving the necessary static files correctly, not just the REST endpoints.
If I find an answer I'll update this.