ipa: Initialize the handlers if they are null

This avoids classes calling >>#initialize on load to initialize
the instance variable. It is a bit sad. If we would just return
handlers GST could optimize the method better (e.g. avoid calling
it and just return the variable).

We should call it initialize handlers or such. This can be done
in a follow up method.

ipa/IPAProtoHandler.st

@@ -38,7 +38,7 @@ Object subclass: IPAProtoHandler [
     ]
 
     IPAProtoHandler class >> handlers [
-        ^ handlers
+        ^ handlers ifNil: [self initialize. handlers].
     ]
 
     registerOn: aDispatcher [