@Test void getTraces_doesntReturnEmptyElements() throws Exception {
    storage.accept(TestObjects.TRACE).execute();

    assertThat(adapter.getTraces(List.of()).execute())
      .isEmpty();

    assertThat(adapter.getTraces(List.of("1")).execute())
      .isEmpty();

    assertThat(adapter.getTraces(List.of("1", "2")).execute())
      .isEmpty();

    assertThat(adapter.getTraces(List.of("1", TestObjects.TRACE.get(0).traceId(), "3")).execute())
      .containsExactly(TestObjects.TRACE);
  }
The contract for {@link zipkin2.storage.SpanStore#getTrace(java.lang.String)} is to return
empty on not found. This ensures a list of results aren't padded with empty ones.
zipkin/src/test/java/zipkin2/internal/TracesAdapterTest.java