fix(jans-auth-server): NPE during client registration

[yuriyz]

Describe the bug

Registration with following payload leads to NPE

{
  "subject_type" : "pairwise",
  "application_type" : "web",
  "scope" : "openid profile address email phone user_name",
  "minimum_acr_priority_list" : [ ],
  "redirect_uris" : [ "https://jenkins-build.jans.io/jans-auth-rp/home.htm", "https://client.example.com/cb", "https://client.example.com/cb1", "https://client.example.com/cb2" ],
  "sector_identifier_uri" : "https://jenkins-build.jans.io/jans-auth/sectoridentifier/a55ede29-8f5a-461d-b06e-76caee8d40b5",
  "client_name" : "jans test app",
  "additional_audience" : [ ],
  "response_types" : [ "code", "id_token" ]
}

2023-07-17 11:46:59,727 ERROR [qtp1072410641-19] 014fbe60-559d-4f61-b157-e1a296017c02 [server.register.ws.rs.action.RegisterCreateAction] (RegisterCreateAction.java:190) - null
java.lang.NullPointerException: null
        at io.jans.as.server.register.ws.rs.RegisterValidator.validateIssuer(RegisterValidator.java:258) ~[classes/:?]
        at io.jans.as.server.register.ws.rs.RegisterValidator.validateSoftwareStatement(RegisterValidator.java:228) ~[classes/:?]
        at io.jans.as.server.register.ws.rs.action.RegisterCreateAction.createClient(RegisterCreateAction.java:102) ~[classes/:?]
        at io.jans.as.server.register.ws.rs.RegisterRestWebServiceImpl.requestRegister(RegisterRestWebServiceImpl.java:52) ~[classes/:?]
        at io.jans.as.server.register.ws.rs.RegisterRestWebServiceImpl$Proxy$_$$_WeldClientProxy.requestRegister(Unknown Source) ~[classes/:?]
        at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]

To Reproduce
Try to register client with payload specified above.