Manual assets not generated when no files specified

[qooban]

1. When you use such configuration:

     "manual": {
       "index": "./manual/index.md",
       "asset": "./manual/asset",
       "files": ["./manual/overview.md"]
     },

   Both - assets and files are copied to the output site. That's OK.

2. When you use such configuration:

     "manual": {
       "index": "./manual/index.md",
       "asset": "./manual/asset"
     },

   esdoc will throw an error:

   assert.js:85
   throw new assert.AssertionError({
   ^
   AssertionError: undefined == true
   at Plugin._setDefault (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc-integrate-manual-plugin\src\Plugin.js:23:5)
   at Plugin._exec (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc-integrate-manual-plugin\src\Plugin.js:14:10)
   at Plugin.onHandleDocs (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc-integrate-manual-plugin\src\Plugin.js:10:10)
   at Plugin._execHandler (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc\out\src\Plugin\Plugin.js:55:26)
   at Plugin.onHandleDocs (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc\out\src\Plugin\Plugin.js:135:10)
   at Function.generate (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc\out\src\ESDoc.js:133:32)
   at ESDocCLI.exec (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc\out\src\ESDocCLI.js:71:23)
   at Object.<anonymous> (C:\Users\jni\WebstormProjects\cloudify-stage\node_modules\esdoc\out\src\ESDocCLI.js:182:7)
   at Module._compile (module.js:570:32)
   at Object.Module._extensions..js (module.js:579:10)

   I would expect that such configuration will copy assets from the directory I specified to manual/asset. I would like to use static content in index.

3. When you use such configuration:

     "manual": {
       "index": "./manual/index.md",
       "asset": "./manual/asset",
       "files": [ ]
     },

   esdoc will generate output but without assets.