Make sure externals is undefined or false for client compilation

[timneutkens]

Currently client compilation or serverless target will have:

[
      // When the serverless target is used all node_modules will be compiled into the output bundles
      // So that the serverless bundles have 0 runtime dependencies
      'amp-toolbox-optimizer' // except this one
    ]

However this is not needed for client compilation. Meaning it should be set to undefined or false to opt-out of adding the ExternalsPlugin:

https://github.com/webpack/webpack/blob/7ed1a8f/lib/WebpackOptionsApply.js#L206-L212