Fix the inconsistent error log

[anujgoyal1]

if (seenContexts.add(context) && !ConfigServiceUtils.isNull(config)) {
        contextSpecificConfigList
            .add(ContextSpecificConfig.newBuilder().setContext(context).setConfig(config).build());
}

in getAllConfig method of DocumentConfigStore, if value of config is null, an error is thrown.
But deleteConfig itself sets the value config in of deletedVersion to null.
So if deleteConfig is called followed by getAllConfigs, it's end up throwing error.