refactor(config): strict null checks (#15291)

lib/config/__snapshots__/migration.spec.ts.snap

lib/config/decrypt.spec.ts

lib/config/defaults.ts

lib/config/index.ts

lib/config/migration.spec.ts

lib/config/migration.ts

lib/config/options/index.spec.ts

lib/config/presets/bitbucket-server/index.ts

lib/config/presets/gitea/index.ts

lib/config/presets/github/index.ts

lib/config/presets/gitlab/index.spec.ts

lib/config/presets/gitlab/index.ts

lib/config/presets/index.spec.ts

lib/config/presets/index.ts

lib/config/presets/internal/index.spec.ts

lib/config/presets/local/common.ts

lib/config/presets/local/index.ts

lib/config/presets/npm/index.ts

lib/config/presets/types.ts

lib/config/presets/util.spec.ts