[Java][jersey2] Fix RuntimeException when HTTP signature parameters are not configured

Created by: sebastien-rosset

This PR fixes a RuntimeException when the HTTP signature parameters are not configured:

java.lang.RuntimeException: Authentication undefined: http_signature
	at ....client.ApiClient.updateParamsForAuth(ApiClient.java:1228)
	at ....client.ApiClient.invokeAPI(ApiClient.java:1047)

Also, add some code comments.

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