Fix compile error due to previous polish

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/invoke/convert/ConversionServiceParameterValueMapperTests.java

@@ -114,8 +114,8 @@ public class ConversionServiceParameterValueMapperTests {
 		}
 
 		@Override
-		public boolean isNullable() {
-			return true;
+		public boolean isMandatory() {
+			return false;
 		}
 
 	}