[FIX] uom: prevent set value of ratio as zero in uom

ZeroDivsionError: float division to Zero is generated because for assigning
value to 'factor' of uom in method '_set_ratio', it tries to divide value by
Zero for uom_type 'Bigger'.

Steps to reproduce:
1) Install 'Inventory' module.
2) Click on 'Configuration' > 'Settings'.
3) Activate 'Units of Measure' > click on 'Units of Measure' button under it.
4) Click on 'Create' button to create new uom category.
5) Give any name > in 'Units of Measure' page, click on 'Add a line'.
6) Enter name and select type as 'Reference unit of measure for this category'.
7) Now, add another line and select type as 'Bigger than the the Reference unit
   of measure' and set the 'ratio' value to Zero, Error will be generated.

By applying this,it will check for the value of 'ratio' to prevent division by 0

sentry - 4174584503

closes odoo/odoo#121409

Signed-off-by: William Henrotin (whe) <whe@odoo.com>