tls binding should handle null key returned from vault

[jfallows]

Describe the bug
When zilla.yaml is misconfigured or keystore is missing a specific key entry then the vault can return null for the PrivateKeyEntry causing an exception.

To Reproduce
Steps to reproduce the behavior:

1. Checkout zilla-examples
2. Go to tls.echo example
3. Change tls binding options keys from localhost to something else like localhostx
4. Run ./setup.sh
5. See error in docker logs

Expected behavior
Zilla should report the error.

Additional context
Docker log output

---
name: example
vaults:
  server:
    type: filesystem
    options:
      keys:
        store: tls/localhost.p12
        type: pkcs12
        password: ${{env.KEYSTORE_PASSWORD}}
bindings:
  tcp_server0:
    type: tcp
    kind: server
    options:
      host: 0.0.0.0
      port: 23456
    exit: tls_server0
  tls_server0:
    type: tls
    kind: server
    vault: server
    options:
      keys:
        - localhostx
      sni:
        - localhost
      alpn:
        - echo
    routes:
      - when:
          - alpn: echo
        exit: echo_server0
  echo_server0:
    type: echo
    kind: server
error
java.util.concurrent.CompletionException: java.lang.NullPointerException: invalid null input
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:332)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:347)
at java.base/java.util.concurrent.CompletableFuture$BiRelay.tryFire(CompletableFuture.java:1498)
at java.base/java.util.concurrent.CompletableFuture$CoCompletion.tryFire(CompletableFuture.java:1219)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2162)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.NamespaceTask.run(NamespaceTask.java:53)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.DispatchAgent.onSystemSignal(DispatchAgent.java:905)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.DispatchAgent.onSystemMessage(DispatchAgent.java:855)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.DispatchAgent.handleRead(DispatchAgent.java:1001)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.concurent.ManyToOneRingBuffer.read(ManyToOneRingBuffer.java:181)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.DispatchAgent.doWork(DispatchAgent.java:675)
at org.agrona.core/org.agrona.concurrent.AgentRunner.doDutyCycle(AgentRunner.java:291)
at org.agrona.core/org.agrona.concurrent.AgentRunner.run(AgentRunner.java:164)
at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: java.lang.NullPointerException: invalid null input
at java.base/java.security.KeyStore.setEntry(KeyStore.java:1580)
at [email protected]/io.aklivity.zilla.runtime.binding.tls.internal.config.TlsBindingConfig.newKeys(TlsBindingConfig.java:394)
at [email protected]/io.aklivity.zilla.runtime.binding.tls.internal.config.TlsBindingConfig.init(TlsBindingConfig.java:91)
at [email protected]/io.aklivity.zilla.runtime.binding.tls.internal.stream.TlsServerFactory.attach(TlsServerFactory.java:216)
at [email protected]/io.aklivity.zilla.runtime.binding.tls.internal.TlsBindingContext.attach(TlsBindingContext.java:57)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.BindingRegistry.attach(BindingRegistry.java:45)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.NamespaceRegistry.attachBinding(NamespaceRegistry.java:141)
at java.base/java.util.Arrays$ArrayList.forEach(Arrays.java:4204)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.NamespaceRegistry.attach(NamespaceRegistry.java:119)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.ConfigurationRegistry.attachNamespace(ConfigurationRegistry.java:159)
at [email protected]/io.aklivity.zilla.runtime.engine.internal.registry.NamespaceTask.run(NamespaceTask.java:48)
... 8 more