fix(ts): Remove non-null assertion in integrationExternalUserMappings.tsx

[floels]

Goal

The goal of this PR is to get rid of a non-null assertion (!) following an optional chaining operator (?.):

https://github.com/getsentry/sentry/blob/3e90887a3632585b048f607668e20927321a39a0/static/app/views/settings/organizationIntegrations/integrationExternalUserMappings.tsx#L105-L112

Optional chain expressions are designed to return undefined if the optional property is nullish. Using a non-null assertion after an optional chain expression doesn't make logical sense and introduces a type safety hole into the code.

Approach

We get rid of the non-null assertion and simply allow the id string to be undefined in the return value of the sentryNamesMapper prop passed down to the <IntegrationExternalMappings /> child component.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 79.91%. Comparing base (3e90887) to head (dd2b38e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #70307      +/-   ##
==========================================
+ Coverage   76.92%   79.91%   +2.98%     
==========================================
  Files        6673     6503     -170     
  Lines      298101   289883    -8218     
  Branches    49918    49918              
==========================================
+ Hits       229320   231654    +2334     
+ Misses      68158    57817   -10341     
+ Partials      623      412     -211     

Files	Coverage Δ
...ionIntegrations/integrationExternalMappingForm.tsx	92.85% <ø> (ø)
...onIntegrations/integrationExternalUserMappings.tsx	0.00% <0.00%> (ø)

... and 662 files with indirect coverage changes