Use existing API id only when defined

[attila]

This fixes a bug where having any custom data defined in provider data
will trigger a ValidationError because the existing API id is null
or undefined.

A typical use case is when provider data contains API key definitions and the plugin cannot deploy new stages anymore. Existing stages will still work though.

provider:
  name: aws
  runtime: ${opt:runtime, 'nodejs8.10'}
  stage: ${opt:stage}
  region: eu-west-1
  apiGateway:
    apiKeySourceType: HEADER
# [...]

This fixes #177

[jconstance-amplify]

Looks good to me, thank you very much for the improvement!