NPE if try to login with an unknown user

[nicoloboschi]

BUG REPORT

1. Please describe the issue you observed:

• What did you do?

Login with a unkown user

• What did you see instead?

caused by:                                                                                                                                                                                                                                                                                                                                                             │
│ java.lang.NullPointerException                                                                                                                                                                                                                                                                                                                                         │
│     at org.bkvm.auth.AuthManager.login(AuthManager.java:65)                                                                                                                                                                                                                                                                                                            │
│     at org.bkvm.api.resources.LoginResource.login(LoginResource.java:65)