[notify.pushover] Fix 'NoneType' error on data retrieval

[dale3h]

Description:
Previous code causes a 'NoneType' object is not iterable error when no data: config is provided to Pushover.

Related issue (if applicable): fixes #2309

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

Example entry for configuration.yaml (if applicable):

automation:
  alias: Garage Door is Open
  trigger:
    platform: state
    entity_id: sensor.garage_door
    state: 'open'
  action:
    service: notify.pushover
    data:
      title: Garage Door
      message: The garage door is open.
      target: iPhone,MacBookPro
      data:
        url: https://home-assistant.io/
        url_title: Home Assistant
        priority: 1
        timestamp: 1466024112
        sound: siren

Checklist:

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

• Documentation added/updated in home-assistant.io

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[balloob]

This code is exactly the same. Passing in a second argument to get specified the default value if the key is not found.

[balloob]

Looks good! 🐬