NPE on JSON EmptySchema

[shoffmeister]

Whenever the built-in JSON schema parser returns EmptySchema, kwack will throw

❯ java -jar ./target/kwack-0.11.0-jar-with-dependencies.jar --bootstrap-server=localhost:9192 --topic xxx --value-serde=xxx=json:@./EmptySchema.json
SLF4J(I): Connected with provider of type [org.slf4j.reload4j.Reload4jServiceProvider]
log4j:WARN No appenders could be found for logger (io.kcache.kwack.KwackConfig).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
java.lang.NullPointerException: Cannot invoke "io.kcache.kwack.schema.ColumnDef.getColumnType()" because "valueColDef" is null
        at io.kcache.kwack.KwackEngine.initTable(KwackEngine.java:602)
        at io.kcache.kwack.KwackEngine.initTables(KwackEngine.java:585)
        at io.kcache.kwack.KwackEngine.init(KwackEngine.java:208)
        at io.kcache.kwack.KwackMain.call(KwackMain.java:156)
        at io.kcache.kwack.KwackMain.call(KwackMain.java:34)
        at picocli.CommandLine.executeUserObject(CommandLine.java:2045)
        at picocli.CommandLine.access$1500(CommandLine.java:148)
        at picocli.CommandLine$RunLast.executeUserObjectOfLastSubcommandWithSameParent(CommandLine.java:2465)
        at picocli.CommandLine$RunLast.handle(CommandLine.java:2457)
        at picocli.CommandLine$RunLast.handle(CommandLine.java:2419)
        at picocli.CommandLine$AbstractParseResultHandler.execute(CommandLine.java:2277)
        at picocli.CommandLine$RunLast.execute(CommandLine.java:2421)
        at picocli.CommandLine.execute(CommandLine.java:2174)
        at io.kcache.kwack.KwackMain.main(KwackMain.java:297)

where EmptySchema.json this case quite literally is

{
}

I ran into this situation while I was transforming a complex OpenAPI YAML into OpenAPI JSON into JSON Schema.

I am not sure what the best way is to handle this (interactive / evolving) scenario; the (more general) root cause is that JsonTransformer.schemaToColumnDef returns null for schemas it does not handle itself. Perhaps throw an application internal exception, instead of letting pass this through to a NPE?