[openwebnet] calculate position only if startedMovingAt is valid. (#10775)

After OH reboot position remains UNDEF.

Signed-off-by: Massimo Valla <mvcode00@gmail.com>

bundles/org.openhab.binding.openwebnet/README.md

@@ -140,10 +140,10 @@ Devices support some of the following channels:
 For Percent commands and position feedback to work correctly, the `shutterRun` Thing config parameter must be configured equal to the time (in ms) to go from full UP to full DOWN.
 It's possible to enter a value manually or set `shutterRun=AUTO` (default) to calibrate `shutterRun` automatically: in this case a *UP >> DOWN >> Position%* cycle will be performed automatically the first time a Percent command is sent to the shutter.
 
-- if `shutterRun` is not set, or is set to AUTO but calibration has not been performed yet, then position estimation will remain `UNDEFINED`
-- if `shutterRun` is wrongly set higher than the actual runtime, then position estimation will remain `UNDEFINED`: try to reduce shutterRun until you find the right value
+- if `shutterRun` is not set, or is set to `AUTO` but calibration has not been performed yet, then position estimation will remain `UNDEF` (undefined)
+- if `shutterRun` is wrongly set higher than the actual runtime, then position estimation will remain `UNDEF`: try to reduce shutterRun until you find the right value
 - before adding/configuring roller shutter Things it is suggested to have all roller shutters `UP`, otherwise the Percent command won’t work until the roller shutter is fully rolled up
-- if the gateways gets disconnected the binding cannot estimate anymore the shutter positions: just roll the shutter all `UP` or `DOWN` and its position will be estimated again
+- if OH is restarted the binding does not know if a shutter position has changed in the meantime, so its position will be `UNDEF`. Move the shutter all `UP`/`DOWN` to synchronize again its position with the binding
 - the shutter position is estimated based on UP/DOWN timing: an error of ±2% is normal
 
 ## Full Example