Skip npm install / bundle when no helium package is selected

Zeppelin 0.7.0 installs node and npm when it first starts for Helium package.

To be installed, or to failed to be installed due to network timeout, it takes some times.

We can just create empty file when no Helium package is enabled, instead of install npm and build bundle.

See discussion https://github.com/apache/zeppelin/pull/2095#issuecomment-285447619