fix(dashboards): Prevent IndexError for deleted dashboard creators

[sentry]

This PR addresses an IndexError: list index out of range that occurred when fetching dashboard details if the user who created the dashboard had been deleted.

Root Cause:
The DashboardDetailsModelSerializer unconditionally accessed [0] on the result of user_service.serialize_many when populating the createdBy field. If obj.created_by_id was set but the corresponding user no longer existed, user_service.serialize_many would return an empty list, leading to an IndexError.

Solution:

1. Refactor get_attrs: The logic to fetch dashboard creators has been moved from serialize to get_attrs. This allows for bulk fetching of all creator users for a list of dashboards in a single user_service.serialize_many call, eliminating N+1 RPC queries.
2. Safe Access in serialize: The createdBy field now safely retrieves the serialized user from the attrs dictionary. If the user is not found (e.g., because they were deleted), None is returned for createdBy instead of raising an IndexError.

Test Plan:
Added a unit test test_get_created_by_is_none_when_creator_deleted to OrganizationDashboardDetailsGetTest which mocks user_service.serialize_many to return an empty list and asserts that createdBy is None in the response.

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Fixes SENTRY-5TSM

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