fix(severity): Prevent skipping severity score when equal to zero

[lobsterkatie]

This fixes a potential bug in assigning severity scores to new groups wherein the score wouldn't be assigned if it was equal to 0. (Falsiness strikes again!) Since scores are currently handled as strings, this is only a hypothetical for now, but worth fixing in case we do ever treat score as a float instead.

It also adds tests for this case and applies a fix I noticed while writing said tests to an existing test. (It now checks for the severity key actually being missing from group metadata rather than severity being set to None.)

[codecov]

Codecov Report

Merging #56225 (e2dde11) into master (d004a6f) will increase coverage by 0.01%.
Report is 3 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #56225      +/-   ##
==========================================
+ Coverage   78.63%   78.65%   +0.01%     
==========================================
  Files        5078     5078              
  Lines      218353   218284      -69     
  Branches    36962    36953       -9     
==========================================
- Hits       171708   171695      -13     
+ Misses      41104    41044      -60     
- Partials     5541     5545       +4     

Files Changed	Coverage
static/app/actionCreators/organization.tsx	ø
static/app/api.tsx	ø
static/app/components/avatarUploader.tsx	ø
static/app/components/button.tsx	ø
static/app/components/charts/baseChart.tsx	ø
static/app/components/charts/utils.tsx	ø
static/app/components/deprecatedforms/form.tsx	ø
...tatic/app/components/deprecatedforms/formField.tsx	ø
...atic/app/components/deprecatedforms/inputField.tsx	ø
...c/app/components/discover/performanceCardTable.tsx	ø
... and 59 more