Null values cause exceptions

[rtempleton]

testcase1.source.json
{"a":1,"c":null}

testcase1.template.json
{
"b":"&$.a",
"c":"&$.c"
}

Result : {"b":"1","c":{"Exception":"org.lappsgrid.json2json.Json2JsonException: Unknown JsonPath ( $.c )\n\tat org.lappsgrid.json2json.template.JsonPathRefUnit.transform(JsonPathRefUnit.java:72)\n\tat org.lappsgrid.json2json.template.JsonUnit.transform(JsonUnit.java:70)\n\tat org.lappsgrid.json2json.template.JsonUnit.transform(JsonUnit.java:82)\n\tat org.lappsgrid.json2json.template.JsonTemplate.transform(JsonTemplate.java:31)\n\tat org.lappsgrid.json2json.Json2Json.transform(Json2Json.java:34)\n\tat com.hwx.slb.studio2.xform.XformTestCase.testCase1(XformTestCase.java:49)\n\tat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)\n\tat sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)\n\tat sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)\n\tat java.lang.reflect.Method.invoke(Method.java:606)\n\tat org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)\n\tat org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)\n\tat org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)\n\tat org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)\n\tat org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:31)\n\tat org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)\n\tat org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)\n\tat org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)\n\tat org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)\n\tat org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)\n\tat org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)\n\tat org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)\n\tat org.junit.runners.ParentRunner.run(ParentRunner.java:236)\n\tat org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)\n\tat org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)\n\tat org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)\n\tat org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)\n\tat org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)\n\tat org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)\n"}}