Zilla asyncapi.mqtt.kafka.proxy crash on startup with NPE: Cannot read field "values" because the return value of "...AsyncapiServerVariableResolver.resolve(String)" is null

[vordimous]

Describe the bug
Zilla throws an NPE on startup that is related to the defined {port} variable.

java.lang.NullPointerException: Cannot read field "values" because the return value of "io.aklivity.zilla.runtime.binding.asyncapi.internal.model.resolver.AsyncapiServerVariableResolver.resolve(String)" is null
     at java.base/java.util.concurrent.CompletableFuture.reportJoin(CompletableFuture.java:413)
     at java.base/java.util.concurrent.CompletableFuture.join(CompletableFuture.java:2118)
     at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.EngineWorker.attach(EngineWorker.java:934)
     at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.EngineManager.lambda$register$15(EngineManager.java:440)
     at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:212)
     at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1709)
     at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:556)
     at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:546)
     at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
     at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
     at java.base/java.util.stream.ReferencePipeline.reduce(ReferencePipeline.java:677)
     at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.EngineManager.register(EngineManager.java:441)
     at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.EngineManager.register(EngineManager.java:399)
     at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.EngineManager.onPathChanged(EngineManager.java:183)
     ... 8 more

To Reproduce
Steps to reproduce the behavior:

1. Run example:

wget -qO- https://raw.githubusercontent.com/aklivity/zilla-examples/main/startup.sh | sh -s -- asyncapi.mqtt.kafka.proxy -v 0.9.99  -e v14

2. See NPE on startup