Don't raise exception if only one count matrix exists

[yihming]

This is for backward compatibility.

Although the default name of raw count matrix has been changed from raw.count or X to counts, we still want to make the data generated by old versions of PegasusIO can still be loaded.

The rna modality doesn't have this issue, because it by default sets None for cur_matrix. But for other modalities, since they have default key names, it restricts the loading.

For example, Souporcell workflow uses PegasusIO v0.2.10, which is the last version supporting Python 3.6, to generate results.