refactor: more strict null checks (#15140)

.eslintrc.js

lib/modules/datasource/docker/index.ts

lib/modules/datasource/galaxy-collection/index.ts

lib/modules/datasource/galaxy/index.ts

lib/modules/datasource/git-refs/base.ts

lib/modules/datasource/git-refs/index.ts

lib/modules/datasource/git-tags/index.ts

lib/modules/datasource/github-releases/common.ts

lib/modules/datasource/github-releases/index.ts

lib/modules/datasource/github-releases/test/index.ts

lib/modules/datasource/github-tags/index.ts

lib/modules/versioning/distro.ts

lib/modules/versioning/node/schedule.ts

lib/util/emoji.ts

lib/util/git/url.ts

test/http-mock.ts

test/util.ts

tsconfig.strict.json