Allow disabling continuous servos

[giri256]

Fixes #74.

ContinuousServo already inherits the ability to disable PWM through fraction = None, but its throttle setter compares the value with numbers before checking for None. The getter also tries to perform arithmetic on None after the servo is disabled.

This handles None before range validation and returns None when reading the throttle of a disabled servo. The regression test covers disabling and enabling the servo again.

Tests:

• pytest tests -q — 7 passed
• Ruff formatting and lint checks passed

[tannewt]

Thank you!