Not handling skip creation part in case of api key not defined for a stage

[sromic]

Let say I have multiple stage and I just want API key added for some/not all stages, e.g.

  apiKeys:
    prod:
      - name: someProdAPIKey
        deleteAtRemoval: false
        value: nonHackableAPIKey

and when I try to deploy to non "prod" stage, I see this line printed as it should be:
"No ApiKey names specified for stage dev so skipping creation" https://github.com/rrahul963/serverless-add-api-key/blob/51201b99103c4d11b431d5da27172bf9919981db/src/helper.js#L296

But plugin continues with a key association process which of course fails with:
" TypeError: apiKeys is not iterable" as there is no apiKeys defined for other stage.

[sromic]

Fixed with #47