18763 FIX ups_in_voltage, ups_out_voltage: Fix crash on non-numeric voltage

Filter phases whose voltage is not an integer in the parse function, so
neither discovery nor the check crashes in int(''), int('NULL') and the
like.

CMK-36324
CMK-36325

Change-Id: I85bbfbbbff95646f5c14ad0209320d32eb9ced4e