fix: AttributeError in collaborator emails when submitter is deleted

[JacobCoffee]

Fixes issue where submitter is none

Closes #19338

[JacobCoffee]

similar issues would occur in these files i think

.id access:

• https://github.com/pypi/warehouse/blob/main/warehouse/accounts/views.py#L1269 -
  str(submitter_user.id) in org role accept

.username access:

• https://github.com/pypi/warehouse/blob/main/warehouse/accounts/views.py#L1393 -
  submitter_user.username in project role decline
• https://github.com/pypi/warehouse/blob/main/warehouse/email/__init__.py#L769 -
  send_collaborator_removed_email also uses submitter.username

#19340