Unexpected behavior: Nextclade ignores CDS unlinked to gene if gene is in gff3

I'm not sure if this is really a bug or just nextclade not behaving as I would expect from the [documentation](https://docs.nextstrain.org/projects/nextclade/en/latest/user/input-files/03-genome-annotation.html). @chaoran-chen created gene maps for the H5N1 segment M as follows: ``` . . CDS 26 784 . + . gene="M1" . . gene 26 1007 . + . gene=M2;ID=gene-M2 . . CDS 26 51 . + . gene=M2;ID=cds-M2;Parent=gene-M2 . . CDS 740 1007 . + . gene=M2;ID=cds-M2;Parent=gene-M2 ``` From the documentation this looked accurate to me. However, I now realized that this leads nextclade to ignore the M1 gene:  Removing the second line (with gene) resolved the issue: ``` . . CDS 26 784 . + . gene="M1" . . CDS 26 51 . + . gene=M2;ID=cds-M2 . . CDS 740 1007 . + . gene=M2;ID=cds-M2 ```  My naive assumption is that nextclade ignores the CDS that is not linked to a gene if a gene is present - however I would not expect this behavior from the docs. This can be tested with examples here: https://github.com/nextstrain/nextclade_data/pull/217