404 error not handled properly in gitlab_replository_file with overwrite_on_create

There is a bug in the overwrite_on_create argument to the gitlab_replository_file resource.

When set to true the provider makes a GET call to check that if the file already exists and checks for errors, but there is no error handling for 404s, which should be expected in some cases when it's creating the file for the first time.

When overwrite_on_create is true and the file does not already exist, an error like this will cause the apply to fail

Error: GET https://gitlab.com/api/v4/projects/00000000/repository/files/example: 404 {message: 404 File Not Found}
│ 
│   with gitlab_repository_file.example,
│   on main.tf line 95, in resource "gitlab_repository_file" "example":
│   95: resource "gitlab_repository_file" "example" {