Serialising null chars was broken

The recent changes that were put in to allow chars to be serialised at
all failed to take into account nullability and thus if a char was null
and that object was first serlialised then desierliased it would throw
a NPE

core/src/main/kotlin/net/corda/core/serialization/amqp/PropertySerializer.kt

@@ -113,13 +113,16 @@ sealed class PropertySerializer(val name: String, val readMethod: Method, val re
      * casting back to a char otherwise it's treated as an Integer and a TypeMismatch occurs
      */
     class AMQPCharPropertySerializer(name: String, readMethod: Method) :
-            PropertySerializer(name, readMethod, Char::class.java) {
+            PropertySerializer(name, readMethod, Character::class.java) {
         override fun writeClassInfo(output: SerializationOutput) {}
 
-        override fun readProperty(obj: Any?, schema: Schema, input: DeserializationInput) = (obj as Int).toChar()
+        override fun readProperty(obj: Any?, schema: Schema, input: DeserializationInput): Any? {
+            return if(obj == null) null else (obj as Int).toChar()
+        }
 
         override fun writeProperty(obj: Any?, data: Data, output: SerializationOutput) {
-            data.putChar((readMethod.invoke(obj) as Char).toInt())
+            val input = readMethod.invoke(obj)
+            if (input != null) data.putChar((input as Char).toInt()) else data.putNull()
         }
     }
 }

core/src/test/kotlin/net/corda/core/serialization/amqp/DeserializeSimpleTypesTests.kt

@@ -50,6 +50,17 @@ class DeserializeSimpleTypesTests {
         assertEquals(c.c, deserializedC.c)
     }
 
+    @Test
+    fun testNullCharacter() {
+        data class C(val c: Char?)
+
+        val c = C(null)
+        val serialisedC = SerializationOutput().serialize(c)
+        val deserializedC = DeserializationInput().deserialize(serialisedC)
+
+        assertEquals(c.c, deserializedC.c)
+    }
+
     @Test
     fun testArrayOfInt() {
         class IA(val ia: Array<Int>)