Throw exception when flat object is null

The existing behavior throws an exception on null flat object by
advancing the parser beyond the end of the flat object input.

We could simply skip a null flat_object field, but this would be
a behavioral change from 2.7.

Signed-off-by: Michael Froh <[email protected]>

Author: msfroh

server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java

@@ -568,6 +568,10 @@ protected void parseCreateField(ParseContext context) throws IOException {
         if (context.externalValueSet()) {
             String value = context.externalValue().toString();
             parseValueAddFields(context, value, fieldType().name());
+        } else if (context.parser().currentToken() == XContentParser.Token.VALUE_NULL) {
+            context.parser().nextToken(); // This triggers an exception in DocumentParser.
+            // We could remove the above nextToken() call to skip the null value, but the existing
+            // behavior (since 2.7) throws the exception.
         } else {
             JsonToStringXContentParser jsonToStringParser = new JsonToStringXContentParser(
                 NamedXContentRegistry.EMPTY,
@@ -594,7 +598,6 @@ protected void parseCreateField(ParseContext context) throws IOException {
                 }
 
             }
-
         }
 
     }