fix(tests): Construct valid command in MockTextDocumentService

[FlorianKroiss]

When executing tests locally, one can observe this error, because the constructor of Command expects a non-null String

  06:46:47 SEVERE [org.eclipse.lsp4j.jsonrpc.RemoteEndpoint] Internal error: Property must not be null: command 
  java.lang.NullPointerException: Property must not be null: command
  	at org.eclipse.lsp4j.jsonrpc.util.Preconditions.checkNotNull(Preconditions.java:29)
  	at org.eclipse.lsp4j.Command.<init>(Command.java:56)
  	at org.eclipse.lsp4e.tests.mock.MockTextDocumentService.codeLens(MockTextDocumentService.java:219)
  	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
  	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
  	at org.eclipse.lsp4j.jsonrpc.services.GenericEndpoint.lambda$recursiveFindRpcMethods$0(GenericEndpoint.java:65)
  	at org.eclipse.lsp4j.jsonrpc.services.GenericEndpoint.request(GenericEndpoint.java:128)
  	at org.eclipse.lsp4j.jsonrpc.RemoteEndpoint.handleRequest(RemoteEndpoint.java:265)
  	at org.eclipse.lsp4j.jsonrpc.RemoteEndpoint.consume(RemoteEndpoint.java:195)
  	at org.eclipse.lsp4j.jsonrpc.json.StreamMessageProducer.handleMessage(StreamMessageProducer.java:189)
  	at org.eclipse.lsp4j.jsonrpc.json.StreamMessageProducer.listen(StreamMessageProducer.java:97)
  	at org.eclipse.lsp4j.jsonrpc.json.ConcurrentMessageProcessor.run(ConcurrentMessageProcessor.java:97)
  	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:572)
  	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
  	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
  	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
  	at java.base/java.lang.Thread.run(Thread.java:1583)