karma-webpack: Webpack 2.2.0-rc.4 throws an error because karma-webpack passes incorrect entry option

I’m submitting a bug report

Webpack version:

2.2.0-rc.4

Webpack Karma version:

1.8.1

Karma version:

1.3.0

Please tell us about your environment:

OSX 10.12

Current behavior:

11 01 2017 17:47:50.284:ERROR [preprocess]: Can not load "webpack"!
  WebpackOptionsValidationError: Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration.entry should be one of these:
   object { <key>: non-empty string | [non-empty string] } | non-empty string | [non-empty string] | function
   The entry point(s) of the compilation.
   Details:
    * configuration.entry should NOT have less than 1 properties ({
        "keyword": "minProperties",
        "dataPath": ".entry",
        "schemaPath": "#/oneOf/0/minProperties",
        "params": {
          "limit": 1
        },
        "message": "should NOT have less than 1 properties",
        "schema": 1,
        "parentSchema": {
          "minProperties": 1,
          "additionalProperties": {
            "oneOf": [
              {
                "description": "The string is resolved to a module which is loaded upon startup.",
                "minLength": 1,
                "type": "string"
              },
              {
                "description": "All modules are loaded upon startup. The last one is exported.",
                "$ref": "#/definitions/common.nonEmptyArrayOfUniqueStringValues"
              }
            ]
          },
          "description": "Multiple entry bundles are created. The key is the chunk name. The value can be a string or an array.",
          "type": "object"
        },
        "data": {}
      }).
      object { <key>: non-empty string | [non-empty string] }
      Multiple entry bundles are created. The key is the chunk name. The value can be a string or an array.
    * configuration.entry should be a string.
    * configuration.entry should be an array:
      [non-empty string]
    * configuration.entry should be an instance of function
      function returning an entry object or a promise..
    at webpack (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/webpack/lib/webpack.js:20:9)
    at new Plugin (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/karma-webpack/lib/karma-webpack.js:63:18)
    at invoke (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/di/lib/injector.js:75:15)
    at Array.instantiate (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/di/lib/injector.js:59:20)
    at get (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/di/lib/injector.js:48:43)
    at /www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/di/lib/injector.js:71:14
    at Array.map (native)
    at Array.invoke (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/di/lib/injector.js:70:31)
    at Injector.get (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/di/lib/injector.js:48:43)
    at instantiatePreprocessor (/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules/karma/lib/preprocessor.js:55:20)
    at Array.forEach (native)

Expected/desired behavior:

It worked fine with Webpack 2.2.0-rc.3. Webpack either hasn’t been validating the entry option or the validation was more loose.

The config which karma-webpack passes to Webpack is this:

{ plugins: [ CKEditorWebpackPlugin { options: [Object] } ],
  module: { rules: [ [Object], [Object] ] },
  resolveLoader:
   { modules:
      [ 'node_modules',
        '/www/ckeditor5/ckeditor5-dev/packages/ckeditor5-dev-tests/node_modules' ] },
  watch: true,
  entry: {},
  output:
   { path: '/_karma_webpack_/',
     publicPath: '/_karma_webpack_/',
     filename: '[name]',
     chunkFilename: '[id].bundle.js' } }

The important part is entry: {}.