refactor: add strict null checks (#14169)

lib/config/decrypt.ts

lib/config/presets/internal/index.ts

lib/config/secrets.ts

lib/config/types.ts

lib/platform/bitbucket-server/types.ts

lib/platform/bitbucket-server/utils.spec.ts

lib/platform/bitbucket-server/utils.ts

lib/platform/bitbucket/comments.ts

lib/platform/github/massage-markdown-links.ts

lib/platform/github/user.ts

lib/platform/types.ts

lib/types/base.ts

lib/types/index.ts

lib/util/git/auth.ts

lib/util/git/author.ts

lib/util/git/types.ts

lib/util/ignore.ts

lib/util/index.spec.ts

lib/util/merge-confidence/index.ts

lib/workers/branch/automerge.ts