private void assertFieldValueNull(Document doc, String fieldName) {
        Field field = doc.getField(fieldName);
        assertNotNull(field);
        FieldValue fieldValue = doc.getFieldValue(field);
        assertNull(fieldValue);
    }
Basic test of JSON streams to Vespa document instances.

@author Steinar Knutsen
@author bratseth
document/src/test/java/com/yahoo/document/json/JsonReaderTestCase.java