Telematic bridge: Fix available topic return null value in topics fie…

…ld when there is no topic available (#569)

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# PR Details
## Description
Telematic bridge sends null topics when there is no topic available. The
idea value for this topics is empty array value [] rather than a null
value. This PR is to initialize the topics value with empty array
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## Related Issue
#570
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## Motivation and Context
telematic testing
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## How Has This Been Tested?
Integration test
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## Types of changes

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- [x] Defect fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that cause existing functionality
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## Checklist:

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- [ ] I have added any new packages to the sonar-scanner.properties file
- [ ] My change requires a change to the documentation.
- [ ] I have updated the documentation accordingly.
- [x] I have read the **CONTRIBUTING** document.
[V2XHUB Contributing
Guide](https://github.com/usdot-fhwa-OPS/V2X-Hub/blob/develop/Contributing.md)
- [ ] I have added tests to cover my changes.
- [ ] All new and existing tests passed.

src/v2i-hub/TelematicBridgePlugin/src/TelematicUnit.cpp

@@ -219,7 +219,7 @@ namespace TelematicBridge
         message[TESTING_TYPE_KEY] = testingType;
         message[EVENT_NAME_KEY] = eventName;
         message[TIMESTAMP_KEY] = duration_cast<milliseconds>(system_clock::now().time_since_epoch()).count();
-        Json::Value topics;
+        Json::Value topics = Json::arrayValue;
         for (const auto &topic : availableTopicList)
         {
             if (!boost::icontains(excludedTopics, topic))

src/v2i-hub/TelematicBridgePlugin/test/test_TelematicUnit.cpp

@@ -43,6 +43,10 @@ namespace TelematicBridge
         auto reply = TelematicUnit::constructAvailableTopicsReplyString(unit, eventLocation, testingType, eventName, topics, excluded_topic);
         auto json = TelematicUnit::parseJson(reply);
         ASSERT_EQ("test_topic", json["topics"][0]["name"].asString());
+
+        reply = TelematicUnit::constructAvailableTopicsReplyString(unit, eventLocation, testingType, eventName, {}, excluded_topic);
+        json = TelematicUnit::parseJson(reply);
+        ASSERT_EQ(1, json["topics"].isArray());
     }
 
     TEST_F(test_TelematicUnit, constructPublishedDataString)