Null check on the payload of KeyValue

The code expect that the payload is never null but there are now a lot
of public constructor functions for KeyValue that don't check for this
property.

Author: Julow

srcs/juloo.keyboard2/KeyValue.java

@@ -291,6 +291,8 @@ public String toString()
 
   private KeyValue(Object p, int kind, int value, int flags)
   {
+    if (p == null)
+      throw new NullPointerException("KeyValue payload cannot be null");
     _payload = p;
     _code = (kind & KIND_BITS) | (flags & FLAGS_BITS) | (value & VALUE_BITS);
   }