Fix Issue #9371 - Set default map style if not provided

[bijanvakili]

Tested with Google Maps in this notebook

Closes #9371

Change List

• Added get_default_map_identifier to map_styles.py to determine map identifier key based on map provider
• Extended Deck constructor to choose a default map style for all 3 providers (as DARK is not an appropriate default for Google Maps)
  • Using a "sentinel" value as default to still allow disabling style (using None) if no base map is used.
• Added unit test to test_deck.py to verify default map style for major map providers if none is provided

[coveralls]

coverage: 91.635%. remained the same
when pulling 119da90 on bijanvakili:issue-9371
into 2819e97 on visgl:master.