To facilitate dynamic deployments where a jvmRoute is set e.g. using a system property, an empty jvmRoute attribute should behave the same as having no jvmRoute attribute set.
Hmm... while looking at the code to propose a patch, I found that the StandardEngine constructor actually looks for a "jvmRoute" system property. This enhancement might not need to exist. I checked, and the jvmRoute system property is properly-documented in the "System Properties" page of the configuration reference. It might help to mention this in the <Engine> documentation for jvmRoute. Changing to a documentation bug.
Fixed in r1628984. Will be in Tomcat 8.0.15's documentation.
Fixed in 7.0.x branch in r1628985. Will be in Tomcat 7.0.57's documentation.