GUI: Jaws OPI: Prevent error when only 2 of 4 axes defined

When using a 4 axis jaws OPI and only 2 axes are defined, an error occurs and the following stack trace is generated:

`2018-10-09 15:07:50 ERROR: Error in on East_Blade.`
`Traceback (most recent call last):`
` File "", line 6, in <module>`
`ZeroDivisionError: float division`

`2018-10-09 15:07:50 ERROR: Error in on West_Blade.`
`Traceback (most recent call last):`
` File "<script>", line 4, in <module>`
`ZeroDivisionError: float division`

It is suspected that as the range of motion of the two undefined axes is zero, when trying to update the image of the blades, the `ZeroDivisionError` occurs.