Refactor assertAuthenticateEquals

- Inspect response for status instead of answer, so that the further checks are only done when the response is actually successful
- Shift retrieval of token to behind a null check, to avoid an NullPointerException when calling asText() on the JsonNode
Instantiate default token to empty string instead of null

This avoids an IllegalArgumentException from MultipartEntityBuilder.addTextBody(), when null is passed as parameter. This case occurs when there are no successful authenticate requests before the bootstrap check is called.