Support empty default environment variable values

[webb-ben]

Overview

• Support empty default environment variable values.
• Allow setting pygeoapi url in default pygeoapi image.

Related Issue / discussion

https://github.com/geopython/pygeoapi/issues/1816

Additional information

Dependency policy (RFC2)

• [x] I have ensured that this PR meets RFC2 requirements

Updates to public demo

• [x] I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • [x] https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• [x] I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• [x] I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines