Consider optional empty arrays to be absent when finding missing fields

Closes gh-519

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2017 the original author or authors.
+ * Copyright 2014-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -147,4 +147,14 @@ public class JsonContentHandlerTests {
 		assertThat(missingFields.size(), is(equalTo(0)));
 	}
 
+	@Test
+	public void describedFieldThatIsNotPresentNestedBeneathOptionalArrayThatIsEmptyIsNotConsideredMissing() {
+		List<FieldDescriptor> missingFields = new JsonContentHandler(
+				"{\"outer\":[]}".getBytes())
+						.findMissingFields(Arrays.asList(new FieldDescriptor("outer"),
+								new FieldDescriptor("outer[]").optional(),
+								new FieldDescriptor("outer[].inner")));
+		assertThat(missingFields.size(), is(equalTo(0)));
+	}
+
 }