Change default for concat_characters to False in open_* functions

I wanted to propose that concat_characters be False for open_{dataset,zarr,dataarray}. I'm not sure how often that affects anyone since working with individual character arrays is probably rare, but it's a particularly bad default in genetics. We often represent individual variations as single characters and the concatenation is destructive because we can't invert it when one of the characters is an empty string (which often corresponds to a deletion at a base pair location, and the order of the characters matters).

I also find it to be confusing behavior (e.g. https://github.com/pydata/xarray/issues/4405) since no other arrays are automatically transformed like this when deserialized.

If submit a PR for this, would anybody object?

Locate the open_{dataset,zarr,dataarray} entry points and their existing tests. Read how concat_characters is currently defaulted and test the behavior with individual character arrays, including empty strings. Done means all three functions default concat_characters to False without breaking explicit opt-in behavior.