Karma test run fails unless entry: is specified in webpack config

**I'm submitting a bug report**

**Versions ->**
webpack-plugin **2.0.0-rc.5**
Windows **10**
Node **8.9.4**
NPM **5.6.0**
Webpack **3.10.0**
Karma **2.0.0**
Karma-webpack **2.0.9**
Chrome **63.0.3239.132**
Typescript **2.6.2**

**Current behavior:**
Bare bores webpack.config just for test running.
Without `plugins:[ new AureliaPlugin()]` in the webpack config tests execute as expected.
But once the preceeding plugin is defined I receive error _TypeError: request.replace is not a function_ after running `karma start`
The webpack config above does not have an `entry:` defined. (as it does not need it for Karma).

However if I add and `entry:` with any string (other than an empty string - these error still), such as the path of the only test spec file, or "ImaRandomString". Then the error goes away and tests run again.

But webpack then tries to emit a main module in addition to the spec test, and also spams the console with _ERROR in multi aurelia-webpack-plugin/runtime/empty-entry aurelia-webpack-plugin/runtime/pal-loader-entry ./test/specs.ts Module not found: Error: Can't resolve 'aurelia-webpack-plugin/runtime/empty-entry' in [my project]_

I've tried removing the `entry:` in the webpack config, and specifying some options in the AureliaPlugin ctor but all attempts still yeild the original error and no tests nun.

**Expected/desired behavior:**
It appears webpack-plugin in dependant on the `entry:` config setting in the webpack.config API and should not be ?

Reproduce the Karma run with AureliaPlugin enabled and no webpack entry, using the versions listed in the report. Inspect how the plugin handles a missing entry and verify that the fix allows tests to run without emitting an unwanted main module or reporting the empty-entry resolution error.

Written by the indexing model from the issue text.