Invalid css-blocks options in 'ember-cli-build.js'

Using this addon in a fresh app gives me:
Invalid css-blocks options in 'ember-cli-build.js': Entry must be a string or array. Instead received undefined.

Environment:
```
$ ember -v
ember-cli: 2.18.2
node: 8.12.0
os: linux x64
```

Steps to reproduce:
```
$ ember new glimmer-app -b @glimmer/blueprint --yarn
$ cd glimmer-app && ember install @css-blocks/ember-cli
$ ember s
```

Reproduce the issue with the listed ember new, ember install, and ember s commands, then inspect ember-cli-build.js and the css-blocks addon configuration path. Done means a fresh glimmer blueprint app starts without the reported invalid options error.

Written by the indexing model from the issue text.