refactor: more strict null checks (#15141)

lib/modules/datasource/gitlab-packages/index.ts

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

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

lib/modules/datasource/golang-version/index.ts

lib/modules/datasource/gradle-version/index.ts

lib/modules/datasource/npm/get.ts

lib/modules/datasource/npm/index.ts

lib/modules/datasource/npm/npmrc.ts

lib/modules/datasource/npm/types.ts

lib/modules/datasource/nuget/common.ts

lib/modules/datasource/nuget/index.ts

lib/modules/datasource/nuget/v2.ts

lib/modules/datasource/nuget/v3.ts

lib/modules/datasource/types.ts

tsconfig.strict.json