Bad error message on empty union

What happened?

When given the following conjure:

types:
  definitions:
    default-package: com.palantir.foo.api
    objects:
      Bar:
        union: {}

the following error is emitted:

Exception in thread "main" picocli.CommandLine$ExecutionException: Error while running command (com.palantir.conjure.java.cli.ConjureJavaCli$GenerateCommand@3315d2d7): java.lang.IllegalArgumentException: cannot unindent 1 from 0
        at picocli.CommandLine.execute(CommandLine.java:1051)
        at picocli.CommandLine.access$900(CommandLine.java:142)
        at picocli.CommandLine$RunLast.handle(CommandLine.java:1246)
        at picocli.CommandLine$RunLast.handle(CommandLine.java:1214)
        at picocli.CommandLine$AbstractParseResultHandler.handleParseResult(CommandLine.java:1122)
        at picocli.CommandLine.parseWithHandlers(CommandLine.java:1405)
        at picocli.CommandLine.run(CommandLine.java:1864)
        at picocli.CommandLine.run(CommandLine.java:1794)
        at com.palantir.conjure.java.cli.ConjureJavaCli.main(ConjureJavaCli.java:45)
Caused by: java.lang.IllegalArgumentException: cannot unindent 1 from 0
        at com.squareup.javapoet.Util.checkArgument(Util.java:53)
        at com.squareup.javapoet.CodeWriter.unindent(CodeWriter.java:107)
        at com.squareup.javapoet.CodeWriter.unindent(CodeWriter.java:103)
        at com.squareup.javapoet.TypeSpec.emit(TypeSpec.java:317)
        at com.squareup.javapoet.JavaFile.emit(JavaFile.java:156)
        at com.squareup.javapoet.JavaFile.writeTo(JavaFile.java:76)
        at com.squareup.javapoet.JavaFile.toString(JavaFile.java:175)
        at com.squareup.javapoet.JavaFile.hashCode(JavaFile.java:169)
        at java.base/java.util.HashMap.hash(HashMap.java:339)
        at java.base/java.util.HashMap.put(HashMap.java:607)
        at java.base/java.util.HashSet.add(HashSet.java:220)
        at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
        at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195)
        at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1654)
        at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
        at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
        at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
        at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
        at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
        at com.palantir.conjure.java.types.ObjectGenerator.generateTypes(ObjectGenerator.java:56)
        at com.palantir.conjure.java.types.TypeGenerator.generate(TypeGenerator.java:37)
        at com.palantir.conjure.java.types.TypeGenerator.emit(TypeGenerator.java:51)
        at com.palantir.conjure.java.cli.ConjureJavaCli$GenerateCommand.run(ConjureJavaCli.java:163)
        at picocli.CommandLine.execute(CommandLine.java:1043)
        ... 8 more

This happened while developing a new API spec (the union was left empty while working on a different part of the API).

What did you want to happen?

Either the input is accepted & code generates, or the input is rejected with a better error message.

Reproduce the empty-union YAML with the ConjureJavaCli entry point, then inspect ObjectGenerator.generateTypes and TypeGenerator around the JavaPoet stack trace. Done means the input either generates successfully or is rejected with a clear error instead of the unindent exception.