NaNs in standard errors of LFC

Hey @const-ae, I'm using the `compute_lfc_se` parameter in `test_de` to get the standard error of the log fold changes to run ash/mash.

In my results I see quite a few genes where the output `lfc_se` is equal to NaN. Is this expected behavior? In what cases does this happens? I couldn't quite figure it out from the code or documentation.