Fix getting progress when there are no strings

[alexkiro]

Unlike COUNT, SUM will actually return NULL not 0 when there are no rows, so we need to coalesce to 0 to avoid such cases.

This issued caused both the progress and status values to be incorrect for translations. Since 0 != None an empty page will have its translations marked as "waiting for translations."

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.07%. Comparing base (caf14b8) to head (7c99102).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #829   +/-   ##
=======================================
  Coverage   93.07%   93.07%           
=======================================
  Files          47       47           
  Lines        4144     4145    +1     
  Branches      704      704           
=======================================
+ Hits         3857     3858    +1     
  Misses        173      173           
  Partials      114      114           

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[zerolab]

Thank you @alexkiro