Add null servlet context check

[tylerbenson]

java.lang.NullPointerException
	at datadog.trace.instrumentation.servlet3.Servlet3Decorator.onContext(Servlet3Decorator.java:81)
	at datadog.trace.instrumentation.servlet3.Servlet3Decorator.onRequest(Servlet3Decorator.java:68)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:782)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:755)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:547)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:190)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:485)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
	at org.eclipse.jetty.server.Server.handle(Server.java:500)
	at org.eclipse.jetty.server.HttpChannel.lambda$handle$1(HttpChannel.java:383)
	at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:547)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:375)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:270)
	at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:311)
	at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:103)
	at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:117)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:806)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:938)
	at java.base/java.lang.Thread.run(Thread.java:844)

[dougqh]

I think the comment is good. I'm wondering if we should add a test as well. Even if it is a bit artificial.

I think we also need to keep expanding our server smoke tests, but that's separate from this PR.

[tylerbenson]

I've added a test that fails without that change.

[tylerbenson]

Key difference is ServletHandler vs ServletContextHandler used in the main set of tests.