Fix NoData values for Coverage JSON output

[geographika]

Overview

As seen during the Diving into pygeoapi workshop the JSON coverage output was broken due to NaN values in the output.

This PR ensures NaN are converted to Python None values, which are then converted to null values in the JSON output, using the same approach as in the xarray driver - https://github.com/geopython/pygeoapi/blob/master/pygeoapi/provider/xarray_.py#L395-L398

Dependency policy (RFC2)

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

Contributions and licensing

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

• I'd like to contribute this bugfix to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines