fix KeyError when append_dim is not None in to_zarr for empty datastore

[luoq]

This bug is introduced in commit 1c5adc9

This should fix issue #4261

[shoyer]

Thanks @luoq. Would it be possible to write test for this to ensure that we don't accidentally regress on this behavior again?

Also: #4035 touches this same code, and it would be nice to know if it fixes it or not.

[luoq]

It seems #4035 also fixes the issue Stephan Hoyer <[email protected]> 于2020年10月25日周日 上午3:16写道：

…

Thanks @luoq <https://github.com/luoq>. Would it be possible to write test for this to ensure that we don't accidentally regress on this behavior again? Also: #4035 <#4035> touches this same code, and it would be nice to know if it fixes it or not. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#4531 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAHT7KDSFKQ4BHNYM3IULCLSMMRYDANCNFSM4S5QKTTA> .

[dcherian]

should be fixed by #4035