Prevent AbstractTypeMapper NPE

Prevent an NPE caused by AbstractTypeMapper returning "null", that is
abstaining from the type decision.

This is useful e.g. to have an optional type discriminator present which
is generated by the system but not needed on reparsing if parse is
called with the correct type known by other means.

Add test.

src/main/java/org/svenson/AbstractTypeMapper.java

@@ -16,7 +16,8 @@ public abstract class AbstractTypeMapper implements TypeMapper
 
     protected Object getPropertyValueFromTokenStream(JSONTokenizer tokenizer, String propertyName, Token first)
     {
-        return getPropertyTokenFromTokenStream(tokenizer, propertyName, first).value();
+        Token token = getPropertyTokenFromTokenStream(tokenizer, propertyName, first);
+        return token != null ? token.value() : null;
     }
     protected Token getPropertyTokenFromTokenStream(JSONTokenizer tokenizer, String propertyName, Token first)
     {

src/test/java/org/svenson/TypeMapperNeutralityTestCase.java

@@ -0,0 +1,34 @@
+package org.svenson;
+
+import org.junit.Test;
+import org.svenson.matcher.PrefixPathMatcher;
+import org.svenson.matcher.TruePathMatcher;
+
+/**
+ * Created by sven on 07.10.15.
+ */
+public class TypeMapperNeutralityTestCase
+{
+
+    @Test
+    public void testName() throws Exception
+    {
+        JSONParser parser = new JSONParser();
+        Mapper typeMapper = new Mapper();
+        typeMapper.setPathMatcher(new TruePathMatcher());
+
+        parser.setTypeMapper(typeMapper);
+
+        parser.parse("{\"foo\":1}");
+
+    }
+
+    private static class Mapper extends AbstractPropertyValueBasedTypeMapper
+    {
+        @Override
+        protected Class getTypeHintFromTypeProperty(Object value) throws IllegalStateException
+        {
+            return null;
+        }
+    }
+}