validation missing for flex fields on discovery rule page

relates to the ZBX-6366 logically
in this case the issue is happening on discovery rule page

validation for flex fields will not happen neither will refresh to the 50s delay for flex period
not sure if that 50s refresh back is relevant, but in any case the flex fields will not validate (since we should keep user's pre-entered value even if it is not correct)

it is fixed on ZBX-6366 for item prototype
and it is already working for templated items and host items

this issue relates to the trunk mainly, not sure regards earlier versions