Fix 0 values not being recognized in axis_twist_compensation

[synthead]

When following the Axis Twist Compensation doc, after calling AXIS_TWIST_COMPENSATION_CALIBRATE, I encountered an error that states:

AXIS_TWIST_COMPENSATION for X axis requires calibrate_start_x, calibrate_end_x and calibrate_y to be defined

Following the configuration reference, I set my values like so:

[axis_twist_compensation]
speed: 2000
# X-axis compensation
calibrate_start_x: 0
calibrate_end_x: 230
calibrate_y: 115
# Y-axis compensation
calibrate_start_y: 0
calibrate_end_y: 181
calibrate_x: 115

This works for me, since the 0 X position and the 0 Y position both have my BL Touch sensor hovering over my print bed. However, I still encountered the error back from the AXIS_TWIST_COMPENSATION GCode from above.

This bug is triggered from checks like these, because 0 makes all() return False:

https://github.com/Klipper3d/klipper/blob/fec3e685c92ef263a829a73510c74245d7772c03/klippy/extras/axis_twist_compensation.py?plain=1#L180-L190

For example:

In [1]: all([None, 1])
Out[1]: False

In [2]: all([0, 1])
Out[2]: False

In [3]: all([0.1, 1])
Out[3]: True

In [4]: all([1, 1])
Out[4]: True

This PR replaces the all() calls with explicit is None checks to see if the value really is not present (as opposed to a value that casts to True). I opted to use a few is None checks as opposed to building an array and enumerating with a for loop because the code is smaller and it reads better :+1:

[JamesH1978]

Thankyou for submitting a PR, please be aware you need to sign off, as per point 3 in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md#what-to-expect-in-a-review

Thanks James

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