access_token: fix possible panic upon deletion

Created by: unknwon

When the Sourcegraph instance is configured to send emails, delete an access token via "byToken" would cause the server to panic because of a nil pointer reference.

Created by: codecov[bot]

Codecov Report

Merging #17150 (d74310e) into main (6234938) will decrease coverage by 0.00%. The diff coverage is 35.71%.

@@            Coverage Diff             @@
##             main   #17150      +/-   ##
==========================================
- Coverage   52.17%   52.16%   -0.01%     
==========================================
  Files        1710     1710              
  Lines       84449    84459      +10     
  Branches     7711     7711              
==========================================
+ Hits        44059    44062       +3     
- Misses      36491    36497       +6     
- Partials     3899     3900       +1     

Flag	Coverage Δ		*Carryforward flag
go	51.31% <35.71%> (-0.01%)
integration	29.86% <ø> (ø)		Carriedforward from 6234938
storybook	28.94% <ø> (ø)		Carriedforward from 6234938
typescript	54.19% <ø> (ø)		Carriedforward from 6234938
unit	35.05% <ø> (ø)		Carriedforward from 6234938

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
internal/db/access_tokens.go	65.25% <33.33%> (-2.01%)
cmd/frontend/graphqlbackend/access_tokens.go	30.00% <37.50%> (-0.53%)
...nal/campaigns/resolvers/changeset_apply_preview.go	59.82% <0.00%> (+0.85%)