[BUG] Button with icon=None raises HTTP 422 error

[krandiash]

mk.gui.Button(..., icon=None, ...)

creates a button correctly, but causes round-trip issues with the backend due to the /store/update/ endpoint being passed a payload of {value: null}, which leads to issues.

Need to debug the /store/update/ to either fix support for null values, or figure out how to handle these values on the frontend.

This issue also happens with other components when passing None values.