(convertNode) Do not create the credentials object if there is nothing to export

[GogoVega]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

When convertNode receives a node with the credentials object, a loop checks for each credential whether it should be exported or not. If no credential is exported the function returns an empty object.

// the node returned
{
  id: "some-id",
  type: "some-type",
  credentials: {}, // <- empty creds object
  ...
}

Which is likely to overwrite unmodified credentials - the object should not be returned.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run npm run test to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality