BUG: pysat.Instrument.empty and xarray

[rstoneback]

Describe the bug
pysat.Instrument.empty checks the length of time or epoch coordinates if the data format is xarray. A dataset may be loaded without one of those which ensures that the Instrument always reports it is empty. This can have other downstream consequences.

To Reproduce
Steps to reproduce the behavior:

1. Load an xarray dataset without 'time' or 'epoch'
2. Check Instrument.empty
3. See error

Expected behavior
empty should be more robust for xarray. Unfortunately xarray does not have its own empty.

Desktop (please complete the following information):

• OS: MacOS 11.5.2
• Version Python 3.8
• Other details about your setup that could be relevant

Additional context
pysat does have a base assumption that data is time indexed. Still, it should do a bit better when data isn't as expected, particularly since pysat doesn't complain at all!!

This bug results in no metadata being assigned. If an instrument reports empty then loaded metadata is not assigned to the Instrument.

[jklenzing]

Closing with merge of #927