Fix Getidentity NPE on nonexistent identity

In both cases `Param.Truster` is a local identity.

First input, things work as expected and the identity is returned:

    FCPPluginMessage
    Identifier=test
    PluginName=plugins.WebOfTrust.WebOfTrust
    Param.Message=GetIdentity
    Param.Truster=uT0HBEQr17txtrkVt~auRBBFUtSubZQWeTAbIgXWMkc
    Param.Identity=wHllqvhRlGLZZrXwqgsFFGbv2V9S33lq~-MTIN2FvOw
    EndMessage

But second input, when the identity in `Param.Identity` does not exist:

    FCPPluginMessage
    Identifier=test
    PluginName=plugins.WebOfTrust.WebOfTrust
    Param.Message=GetIdentity
    Param.Truster=uT0HBEQr17txtrkVt~auRBBFUtSubZQWeTAbIgXWMkc
    Param.Identity=pxtehd-TmfJwyNUAW2Clk4pwv7Nshyg21NNfXcqzFv4
    EndMessage

In `wrapper.log`:

    jvm 1    | Failed to match message: GetIdentity with reply
    jvm 1    | java.lang.NullPointerException
    jvm 1    | 	at plugins.WebOfTrust.fcp.GetIdentity.addIdentityReplyFields(GetIdentity.java:46)
    jvm 1    | 	at plugins.WebOfTrust.fcp.GetIdentity.handle(GetIdentity.java:38)
    jvm 1    | 	at plugins.WebOfTrust.FCPInterface.handle(FCPInterface.java:37)
    jvm 1    | 	at plugins.WebOfTrust.WebOfTrust.handle(WebOfTrust.java:272)
    jvm 1    | 	at freenet.pluginmanager.PluginTalker.sendSyncInternalOnly(PluginTalker.java:75)
    jvm 1    | 	at freenet.pluginmanager.PluginTalker$1.run(PluginTalker.java:61)
    jvm 1    | 	at freenet.support.PooledExecutor$MyThread.innerRun(PooledExecutor.java:247)
    jvm 1    | 	at freenet.support.PooledExecutor$MyThread.realRun(PooledExecutor.java:187)
    jvm 1    | 	at freenet.support.io.NativeThread.run(NativeThread.java:129)

And in FCP:

    FCPPluginReply
    PluginName=plugins.WebOfTrust.WebOfTrust
    Identifier=test
    Replies.Description=Could not match message with reply because of an exception: null
    Replies.Message=Error
    Replies.OriginalMessage=GetIdentity
    EndMessage

This pull request makes the response for a nonexistent Identity be:

    FCPPluginReply
    PluginName=plugins.WebOfTrust.WebOfTrust
    Identifier=test
    Replies.Message=Identity
    EndMessage