Update method of asking XArray not to write _FillValue

[DWesl]

Description

_FillValue = None is now the preferred way to ask XArray not to write fill values

Motivation and Context

This allows the examples to work with default versions of XArray.
Apparently this changed in the few years since I looked at the documentation: see pydata/xarray#1598 for discussion.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I give permission for my code to be distributed under the existing license