Reconfigure branch may not interpolate `$default` base branch correctly

[jamietanna]

When Renovate's reconfigure branch runs, we do not seem to correctly determine what the $default branch is:

DEBUG: Setting current branch to main
DEBUG: latest commit
{
  "branchName": "main",
  "latestCommitDate": "2026-03-03T09:59:19.000Z",
  "sha": "..."
}

DEBUG: findIssue(Dependency Dashboard)
DEBUG: Found issue 3
...
DEBUG: Issues cache: Updating issue 3 in cache
DEBUG: baseBranchPatterns "$default" matches "undefined"
DEBUG: baseBranches
{
  "baseBranches": [
    null,
    "dev-eks"
  ]
}

WARN: Base branch does not exist - skipping
DEBUG: baseBranch: dev-eks
{
  "baseBranch": "dev-eks"
}

This leads to Reconfigure PRs missing important context, and ends up not presenting any information from the default branch.