Upgrade Fails at ALTER TABLE `civicrm_price_field_value` when Amount is empty

The upgrade scriptfor "4.7.28" runs the following:

ALTER TABLE `civicrm_price_field_value` CHANGE `amount` `amount` DECIMAL(18,9) NOT NULL COMMENT 'Price field option amount'

If any of the amounts are empty strings then the query will fail with

 [nativecode=1366 ** Incorrect DECIMAL value: '0' for column '' at row -1]

We had several rows with empty values for the amount. Setting them to 0 allowed the query to complete. 

I ended up running a query to update all rows where the amount = '' to amount = 0.