[WICKET-7173] JavaxWebSocketConfigurator: Add back null check for httpSession

[weber-semedy]

Changes for WICKET-7162 (#1223) removed a null check for httpSession, leading to another NPE when trying to add httpSession to the userProperties map

SEVERE [...] org.apache.catalina.core.StandardWrapperValve.invoke Servlet.service() for servlet [default] in context with path [/contextpath] threw exception [java.lang.NullPointerException] with root cause
	java.lang.NullPointerException
		at java.base/java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011)
		at java.base/java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:1006)
		at org.apache.wicket.protocol.ws.javax.WicketServerEndpointConfig$JavaxWebSocketConfigurator.modifyHandshake(WicketServerEndpointConfig.java:146)
		at org.apache.tomcat.websocket.server.UpgradeUtil.doUpgrade(UpgradeUtil.java:210)
		at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:78)
		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:109)
		at org.apache.wicket.protocol.http.WicketFilter.processRequestCycle(WicketFilter.java:293)

I requested a Jira account to file a new issue and retrieve the id. The id is now added.

NOTE:
We will still see the approproriate (follow-up) exception when httpSession is null:

org.apache.coyote.AbstractProtocol$ConnectionHandler.process Error reading request, ignored
	java.lang.IllegalStateException: There is no HTTP Session bound. Without a session Wicket won't be able to find the stored page to update its components
		at org.apache.wicket.protocol.ws.api.AbstractWebSocketProcessor.<init>(AbstractWebSocketProcessor.java:111)
		at org.apache.wicket.protocol.ws.javax.JavaxWebSocketProcessor.<init>(JavaxWebSocketProcessor.java:48)

[martin-g]

Thank you, @weber-semedy !