fix(gitlab): Prevent nil pointer dereference when HeadPipeline is empty

In this particular example `mr.HeadPipeline.SHA` panics on a nil pointer dereference because HeadPipeline is empty.

This seems to be caused by the lack of permission to update the commit status.

```go
runtime.gopanic
        runtime/panic.go:1038
runtime.panicmem
        runtime/panic.go:221
runtime.sigpanic
        runtime/signal_unix.go:735
github.com/runatlantis/atlantis/server/events/vcs.(*GitlabClient).PullIsMergeable
        github.com/runatlantis/atlantis/server/events/vcs/gitlab_client.go:208
github.com/runatlantis/atlantis/server/events/vcs.(*ClientProxy).PullIsMergeable
        github.com/runatlantis/atlantis/server/events/vcs/proxy.go:72
github.com/runatlantis/atlantis/server/events/vcs.(*pullReqStatusFetcher).FetchPullStatus
        github.com/runatlantis/atlantis/server/events/vcs/pull_status_fetcher.go:28
github.com/runatlantis/atlantis/server/events.(*ApplyCommandRunner).Run
        github.com/runatlantis/atlantis/server/events/apply_command_runner.go:105
github.com/runatlantis/atlantis/server/events.(*DefaultCommandRunner).RunCommentCommand
        github.com/runatlantis/atlantis/server/events/command_runner.go:252
```

The least invasive solution is to simply use the commit-hash from pull and guess that the pipeline was "skipped" unless the HeadPipeline is there.

The outcome is:

When mr.HeadPipeline is present:
- use the commit hash and status from the HeadPipeline
When mr.HeadPipeline is NOT present:
- use the commit hash from pull request struct
- assume the pipeline was "skipped"

In cases where GitLab is configured to require a pipeline to pass, this results on a message saying the MR is not mergeable.

More info:
- runatlantis#1852

scripts/e2e-deps.sh

@@ -18,7 +18,7 @@ unzip terraform_${TERRAFORM_VERSION}_linux_amd64.zip
 chmod +x terraform
 cp terraform /home/circleci/go/bin
 # Download ngrok to create a tunnel to expose atlantis server
-wget https://bin.equinox.io/c/bNyj1mQVY4c/ngrok-v2-stable-linux-amd64.zip -O ngrok-stable-linux-amd64.zip 
+wget https://bin.equinox.io/c/bNyj1mQVY4c/ngrok-v2-stable-linux-amd64.zip -O ngrok-stable-linux-amd64.zip
 unzip ngrok-stable-linux-amd64.zip
 chmod +x ngrok
 # Download jq

server/events/vcs/gitlab_client.go

@@ -280,14 +280,23 @@ func (g *GitlabClient) PullIsMergeable(repo models.Repo, pull models.PullRequest
 		return false, err
 	}
 
+	// Prevent nil pointer error when mr.HeadPipeline is empty
+	// See: https://github.com/runatlantis/atlantis/issues/1852
+	commit := pull.HeadCommit
+	isPipelineSkipped := true
+	if mr.HeadPipeline != nil {
+		commit = mr.HeadPipeline.SHA
+		isPipelineSkipped = mr.HeadPipeline.Status == "skipped"
+	}
+
 	// Get project configuration
 	project, _, err := g.Client.Projects.GetProject(mr.ProjectID, nil)
 	if err != nil {
 		return false, err
 	}
 
 	// Get Commit Statuses
-	statuses, _, err := g.Client.Commits.GetCommitStatuses(mr.ProjectID, mr.HeadPipeline.SHA, nil)
+	statuses, _, err := g.Client.Commits.GetCommitStatuses(mr.ProjectID, commit, nil)
 	if err != nil {
 		return false, err
 	}
@@ -302,7 +311,6 @@ func (g *GitlabClient) PullIsMergeable(repo models.Repo, pull models.PullRequest
 		}
 	}
 
-	isPipelineSkipped := mr.HeadPipeline.Status == "skipped"
 	allowSkippedPipeline := project.AllowMergeOnSkippedPipeline && isPipelineSkipped
 
 	ok, err := g.SupportsDetailedMergeStatus()