Fix gerrit scm coverage

Author: felipecrs

lib/modules/platform/gerrit/scm.ts

@@ -154,13 +154,14 @@ export class GerritScm extends DefaultGitScm {
           // avoid creating a new change if the base branch has changed.
           // updatePr() will take care of moving the existing change to a different base
           // branch if needed.
-          const changeBranch = existingChange?.branch ?? commit.baseBranch!;
+          const changeBranch = existingChange.branch ?? commit.baseBranch!;
           const pushResult = await git.pushCommit({
             sourceRef: commit.branchName,
             targetRef: `refs/for/${changeBranch}`,
             files: commit.files,
             pushOptions,
           });
+          /* v8 ignore else -- should never happen */
           if (pushResult) {
             return commitSha;
           }