Do not call update() in constructor

[fabaff]

Description:

• Do not call update() in constructor.

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: command_line
    name: CPU Temperature
    command: "cat /sys/class/thermal/thermal_zone0/temp"
    unit_of_measurement: "°C"
    value_template: '{{ value | multiply(0.001) }}'

Checklist:

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.

[pvizeli]

Why not STATE_UNKNOWN?

[MartinHjelmare]

It's the base Entity class that should handle converting None to STATE_UNKNOWN.