KAFKA-20693: fix npe in abstract herder when no config value

[gaetancollaud]

Summary

Debezium (and maybe others) has a concept of deprecated fields. Those fields are returned by the config() method but are omitted from validate().

After the validation of a connector fail, AbstractHerder loops over the keys from config() but then tries to get the configValue() returned by validate() this creates a NPE.

Other solution

Instead of checking for null, another fix would be to not add the null ConfigValueInfo in the first place:

// line 980 of AbstractHerder.java
ConfigValueInfo configValueInfo = null;
if (configValueMap.containsKey(configName)) {
    ConfigValue configValue = configValueMap.get(configName);
    configValueInfo = convertConfigValue(configValue, type);
    errorCount += configValue.errorMessages().size();
}
configInfoList.add(new ConfigInfo(configKeyInfo, configValueInfo)); // move this inside the if above

Tests

• I modified SampleSourceConnector to not return all the fields in validate()
• I added testDifferenceInConfigDefAndValidation in AbstractHerderTest