Do not fetch manual variables for GenericCommitStatus

What does this MR do and why?

This came out of a discussion here: #524592 (closed) (internal only)

Fixes fetching of manual variables with graphql. Since GenericCommitStatus does not have an action, the following query was giving us a 500 when the pipeline includes a GenericCommitStatus:

{
  project(fullPath: "lauraXD/bug-testing") {  
    pipelines {  
      nodes {  
        jobs {  
          nodes {  
            manualVariables {  
              nodes {  
                key  
                value  
              }  
            }  
          }  
        }  
      }  
    }  
  }  
}  

To test this, add a GenericCommitStatus to a pipeline and run a pipeline with manual variables. The query should work ☑️

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