Better null safety.

Defense against an exception consistently thrown when running functional tests against 1.532.1 (but not 1.540, for whatever reason):
… hudson.PluginWrapper stop
WARNING: Failed to shut down …
java.lang.NullPointerException
	at jenkins.model.Jenkins.lookup(Jenkins.java:3898)
	at hudson.PluginWrapper.getPlugin(PluginWrapper.java:287)
	at hudson.PluginWrapper.stop(PluginWrapper.java:377)
	at hudson.PluginManager.stop(PluginManager.java:650)
	at org.jvnet.hudson.test.TestPluginManager.reallyStop(TestPluginManager.java:129)
	at org.jvnet.hudson.test.TestPluginManager.access$000(TestPluginManager.java:55)
	at org.jvnet.hudson.test.TestPluginManager$1.run(TestPluginManager.java:140)