No need to check if `repository_ref` is present

There is no need to check if `repository_ref` is present as:
```
      @repository_ref = if repository_ref.present?
                          repository_ref
                        else
                          nil
                        end
```

is as same as doing:

```
@repository_ref = repository_ref
```

lib/gitlab/project_search_results.rb

@@ -4,11 +4,7 @@ module Gitlab
     def initialize(project_id, query, repository_ref = nil)
       @project = Project.find(project_id)
-      @repository_ref = if repository_ref.present?
-                          repository_ref
-                        else
-                          nil
-                        end
+      @repository_ref = repository_ref
       @query = Shellwords.shellescape(query) if query.present?
     end