Stop flagging documented field beneath option null field as missing

Closes spring-projectsgh-557

Author: wilkinsona

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/JsonContentHandler.java

@@ -86,7 +86,7 @@ private boolean isMissing(FieldDescriptor candidate, Object payload) {
 		}
 		ExtractedField extracted = this.fieldProcessor.extract(candidate.getPath(),
 				payload);
-		return isEmptyCollection(extracted.getValue());
+		return extracted.getValue() == null || isEmptyCollection(extracted.getValue());
 	}
 
 	private boolean isEmptyCollection(Object value) {

spring-restdocs-core/src/test/java/org/springframework/restdocs/payload/JsonContentHandlerTests.java

@@ -183,4 +183,13 @@ public void describedMissingFieldThatIsChildOfNestedOptionalArrayThatContainsAnO
 		assertThat(missingFields.get(0).getPath()).isEqualTo("a.[].b.[].c");
 	}
 
+	@Test
+	public void describedMissingFieldThatIsChildOfOptionalObjectThatIsNullIsNotConsideredMissing() {
+		List<FieldDescriptor> missingFields = new JsonContentHandler(
+				"{\"a\":null}".getBytes()).findMissingFields(
+						Arrays.asList(new FieldDescriptor("a").optional(),
+								new FieldDescriptor("a.b")));
+		assertThat(missingFields.size()).isEqualTo(0);
+	}
+
 }