add GIT_STRATEGY empty

What does this MR do?

Add GIT_STRATEGY empty to allow running a job without having runner cache or git repo.

Why was this MR needed?

To close #27499 (closed) and gitlab#17103 (closed)

What's the best way to test this MR?

Create a pipeline with 2 jobs in 2 stages running on same runner, touch file in job1, there shall not be a file in job2

stages:
  - build
  - test1
  - test2
  
build:
  stage: build
  tags:
    - test-runner
  script:
    - touch test1.txt
    - ls -lah

test none:
  stage: test1
  tags:
    - test-runner
  variables:
    GIT_STRATEGY: none
  script:
    - ls -lah

test clean:
  stage: test2
  tags:
    - test-runner
  variables:
    GIT_STRATEGY: empty
  script:
    - ls -lah

What are the relevant issue numbers?

• #27499 (closed)
• gitlab#17103 (closed)