Handle empty config file

[SpacemanPaul]

Reason for this pull request

An empty/blank/pure white space config file triggered an error.

Proposed changes

• Parse empty/blank/pure white space config files as INI files instead of YAML files. This results in the correct high level structure being returned to the Config API (i.e. an empty dictionary instead of None).

• Closes Empty ~/.datacube.conf causes errors #1603

• Tests added / passed

• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.43%. Comparing base (3df96cd) to head (c32c1bc).

Additional details and impacted files

@@             Coverage Diff              @@
##           develop-1.9    #1604   +/-   ##
============================================
  Coverage        85.43%   85.43%           
============================================
  Files              140      140           
  Lines            15496    15496           
============================================
  Hits             13239    13239           
  Misses            2257     2257           

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