Add a test for instruments where they load a time without data

[aburrell]

Is your feature request related to a problem? Please describe.
After adding a new instrument with a custom load function, I learned that the test suite does not try to load times where no data has been downloaded as a part of the standard test suite. This is important to ensure the load return values when no data is present are correct.

Describe the solution you'd like
Test to ensure that load for times without data returns an empty meta object and an empty pandas DataFrame or xarray Dataset.

Describe alternatives you've considered
Ignoring this gap

Additional context
Lines missing coverage in new Madrigal instrument. from this pull request: pysat/pysatMadrigal#89

[jklenzing]

Implementation idea: add a load test with a date prior to the test date.