Fixed Pollen.com bugs with ZIP codes and invalid API responses

[bachya]

Description:

1. Fixes a bug wherein ZIP codes starting with 0 would fail.
2. Adds some guards against bad API calls.

Related issue (if applicable): fixes #12787

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#4784

Example entry for configuration.yaml (if applicable):

sensor:
- platform: pollen
  zip_code: "00544"
  monitored_conditions:
    - allergy_average_forecasted
    - allergy_average_historical
    - allergy_index_today
    - allergy_index_tomorrow
    - allergy_index_yesterday
    - disease_average_forecasted

Checklist:

• The code change is tested and works locally.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass

[balloob]

Awesome 🎉