Fix unhandled exception in Ambient PWS config entry

[bachya]

Description:

If the ambient_station component is set up as a config entry, an exception gets thrown due to a reliance on configuration.yaml values; this PR fixes that.

Related issue (if applicable): fixes #21255

Pull request in home-assistant.io with documentation (if applicable): N/A

Example entry for configuration.yaml (if applicable):

ambient_station:
  api_key: !secret ambient_pws_api_key
  app_key: !secret ambient_pws_app_key

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.