Add missing attributes on token audit events

What does this MR do?

Fix missing attributes on token audit events

The bug is caused by the column value being overwritten will nil value in the details hash. The after_validation callback always writes the column value using the details value, regardless whether it has been specified or not.

This change ensures nil check was performed before setting values in both column and detail hash. The column value takes precedence over the details value.

Before

-[ RECORD 1 ]--+------------------------------------------------------------------------------------------
id             | 15
author_id      | 1
entity_id      | 2
entity_type    | Project
details        | ---                                                                                      +
               | :target_details: alice wonderful token                                                   +
               | :action: :custom                                                                         +
               | :custom_message: 'Created project access token with token_id: 8 with scopes: ["read_api",+
               |   "read_repository"]'                                                                    +
               | :ip_address: 127.0.0.1                                                                   +
               | :entity_path: gitlab-org/gitlab-shell                                                    +
               |
ip_address     | 127.0.0.1
author_name    | [NULL]
entity_path    | gitlab-org/gitlab-shell
target_details | alice wonderful token
created_at     | 2021-06-14 11:30:00.745363
target_type    | [NULL]
target_id      | [NULL]

After

-[ RECORD 1 ]--+------------------------------------------------------------------------------------------
id             | 18
author_id      | 1
entity_id      | 2
entity_type    | Project
details        | ---                                                                                      +
               | :target_id: 9                                                                            +
               | :target_type: PersonalAccessToken                                                        +               
               | :target_details: alice amazing token                                                     +
               | :action: :custom                                                                         +
               | :custom_message: 'Created project access token with token_id: 9 with scopes: ["read_api",+
               |   "read_repository"]'                                                                    +
               | :ip_address: 127.0.0.1                                                                   +
               | :entity_path: gitlab-org/gitlab-shell                                                    +
               | :author_name: Harry Potter                                                               +
               |
ip_address     | 127.0.0.1
author_name    | Harry Potter
entity_path    | gitlab-org/gitlab-shell
target_details | alice amazing token
created_at     | 2021-06-14 11:36:02.180232
target_type    | PersonalAccessToken
target_id      | 9

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #333416 (closed)