FCPInterface.handleSubscribe(): Fix empty reply

All WOT unit tests should succeed now against fred plugin-fcp-rewrite :)
The next step will be to adapt FCPClientReferenceImplementation and
DebugFCPClient to work with plugin-fcp-rewrite as well. After that,
DebugFCPClient shall be used to do an online test of plugin-fcp-rewrite.

FYI the issue was caught by this unit test failure:
java.lang.AssertionError: expected:<Subscribed> but was:<null>
	at org.junit.Assert.fail(Assert.java:88)
	at org.junit.Assert.failNotEquals(Assert.java:743)
	at org.junit.Assert.assertEquals(Assert.java:118)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at plugins.WebOfTrust.SubscriptionManagerFCPTest
		.testSubscribeTo(SubscriptionManagerFCPTest.java:179)
	at plugins.WebOfTrust.SubscriptionManagerFCPTest
		.testSubscribeUnsubscribe(SubscriptionManagerFCPTest.java:149)
	at plugins.WebOfTrust.SubscriptionManagerFCPTest
		.testSubscribeUnsubscribe(SubscriptionManagerFCPTest.java:141)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)