Adding a default value for displaying max_accel_to_decel

[Joldiges]

max_accel_to_decel is being deprecated from Klipper and is being included in fewer and fewer default printer.cfg's accordingly.

The proposed is a hot-fix to allow KlipperLCD to run on machines without max_accel_to_decel in the config.

The implemented uses the get function of the dictionary type so that a default value can be retrieved if it doesn't exist within the dict.

(image of KlipperLCD crashing, without the changes)

(adding a pic of the code, I don't know why :) )

(image showing KlipperLCD not crashing)

[SWRJEEDOM]

Great, it works perfectly