Fix NPE when checking JWT token

Steps to reproduce:

1. Configure an OAuth2 client with:
   • Name : java7 client
   • Client Id : nuxeo-java7-client
   • Client Secret : 8832hrrhoiy382
   • Redirect URIs : nuxeo://authorize
   • Auto-grant : Yes
   • Activated : Yes
2. Run the following command where "assertion" key is missing

   curl --location --request POST 'http://localhost:8080/nuxeo/oauth2/token' \
   --header 'Content-Type: application/x-www-form-urlencoded' \
   -u Administrator:Administrator \
   --data-urlencode 'grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer' \
   --data-urlencode 'client_id=nuxeo-java7-client' \
   --data-urlencode 'client_secret=8832hrrhoiy382'
   

1. Observe that the response corresponds to the redirection to the Nuxeo HTML error page instead, which displays the NPE in the stacktrace

2020-07-09T16:47:05,756 ERROR [nuxeo-error-log] javax.servlet.ServletException: java.lang.NullPointerException
	...
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
	at java.util.Objects.requireNonNull(Objects.java:203)
	at org.nuxeo.ecm.jwt.JWTServiceImpl.verifyToken(JWTServiceImpl.java:231)
	at org.nuxeo.ecm.platform.oauth2.NuxeoOAuth2Servlet.doPostToken(NuxeoOAuth2Servlet.java:338)
	at org.nuxeo.ecm.platform.oauth2.NuxeoOAuth2Servlet.doPost(NuxeoOAuth2Servlet.java:121)
 

Expected behavior:
the error is returned with the JSON response, as with an incorrect "assertion" value

{
  "error_description": "Invalid client: Administrator",
  "error": "invalid_client"
}