@NotNull
  public List<MavenArtifact> findArtifacts(@Nullable String groupId, @Nullable String artifactId) {
    Map<String, List<MavenArtifact>> groupMap = myData.get(groupId);
    if (groupMap == null) return Collections.emptyList();

    List<MavenArtifact> res = groupMap.get(artifactId);
    return res == null ? Collections.emptyList() : res;
  }
@NotNull
  public List<MavenArtifact> findArtifacts(@Nullable String groupId, @Nullable String artifactId)