Importing products cannot be forced to 0

[johndcoy-zz]

Importing existing products to force stock count to 0 does not work.

Steps to reproduce the issue

1. Create a variation with a stock of 1 and another with a stock of 0

2. Export variations.
   
   Image Link: http://cld.wthms.co/ibEAwQ

3. Change stock count for each variation from 1 to 0 and 1 to 0 accordingly
   
   Image Link: http://cld.wthms.co/2YERWk

4. Import changes

Also, altered products to stock count of 0 are forced to backorder status:

Image Link: http://cld.wthms.co/igNsnM

If you export again the sheet still reads as 0 for Backorders Allowed but status will show as backorder for In Stock? column.

Expected/actual behavior

Changes from 0 to 1 worked as expected ( http://cld.wthms.co/ulvetF ) but from 1 to 0 does not have the "Out of Stock" label ( http://cld.wthms.co/lqKXMF )

Isolating the problem

• [ X] This bug happens with only WooCommerce plugin active
• [ X] This bug happens with a default WordPress theme active, or Storefront
• [ X] I can reproduce this bug consistently using the steps above

WordPress Environment

``` Copy and paste the system status report from **WooCommerce > System Status** in WordPress admin here. ```

[claudiulodro]

Duplicate of #19485 and fixed in #19504.