@ppkarwasz I have two questions about #175.

1. Why are there no @NonNull annotations and only @Nullable annotations, e.g., why isn't it public PackageURL(@NonNull String)? I'd prefer to make it so that any private APIs could be marked as @NonNull, at least, since all input for those methods come internally.
2. Previously, a null type or name produced a checked MalformedPackageURLException and now they produce an unchecked NullPointerException which in the tests requires catching NullPointerException which violates, e.g., DCN_NULLPOINTER_EXCEPTION.I think it's best to put back the MalformedPackageURLException in the case of the constructors.