fix(openchallenges): support null value for challenge.platform (SMR-490) (Sage-Bionetworks#3490)

Author: tschaffter

apps/openchallenges/challenge-service/src/main/java/org/sagebionetworks/openchallenges/challenge/service/model/mapper/ChallengeJsonLdMapper.java

@@ -29,7 +29,10 @@ public ChallengeJsonLdDto convertToDto(ChallengeEntity entity, Object... args) {
     if (entity != null) {
       BeanUtils.copyProperties(entity, dto, "stars", "inputDataTypes", "platform", "operation");
       dto.setStatus(ChallengeStatusDto.fromValue(entity.getStatus()));
-      dto.setPlatform(platformMapper.convertToDto(entity.getPlatform()));
+      var platformDto = platformMapper.convertToDto(entity.getPlatform());
+      if (platformDto != null) {
+        dto.setPlatform(platformDto);
+      }
       if (entity.getOperation() != null) {
         dto.setOperation(edamConceptMapper.convertToDto(entity.getOperation()));
       }

apps/openchallenges/challenge-service/src/main/java/org/sagebionetworks/openchallenges/challenge/service/model/mapper/ChallengeMapper.java

@@ -33,7 +33,11 @@ public ChallengeDto convertToDto(ChallengeEntity entity, Object... args) {
       BeanUtils.copyProperties(entity, dto, "stars", "inputDataTypes", "platform", "operation");
       log.trace("challenge dto before set: {}", dto);
       dto.setStatus(ChallengeStatusDto.fromValue(entity.getStatus()));
-      dto.setPlatform(platformMapper.convertToDto(entity.getPlatform()));
+      // Only set the platform when it exists. Avoid serializing an empty object with null fields.
+      var platformDto = platformMapper.convertToDto(entity.getPlatform());
+      if (platformDto != null) {
+        dto.setPlatform(platformDto);
+      }
       if (entity.getOperation() != null) {
         dto.setOperation(edamConceptMapper.convertToDto(entity.getOperation()));
       }

apps/openchallenges/challenge-service/src/main/java/org/sagebionetworks/openchallenges/challenge/service/model/mapper/SimpleChallengePlatformMapper.java

@@ -25,10 +25,12 @@ public SimpleChallengePlatformDto convertToDto(
     SimpleChallengePlatformEntity entity,
     Object... args
   ) {
-    SimpleChallengePlatformDto dto = new SimpleChallengePlatformDto();
-    if (entity != null) {
-      BeanUtils.copyProperties(entity, dto);
+    if (entity == null) {
+      // Preserve null instead of returning an empty object with all fields null.
+      return null;
     }
+    SimpleChallengePlatformDto dto = new SimpleChallengePlatformDto();
+    BeanUtils.copyProperties(entity, dto);
     return dto;
   }
 }