coverage 1.16.0: format_coverage fails on the {} that dart test --coverage writes for fully skipped suites #2667

When every test in a suite is skipped (for example, excluded by tag in dart_test.yaml), dart test --coverage=coverage writes coverage/<path>.vm.json containing {}. Since 1.16.0, HitMap.parseFiles throws on that file, so format_coverage --in=coverage fails after a passing test run:

FormatException: Unrecognized coverage JSON in "coverage/test/ensure_build_test.dart.vm.json". Expected a {"coverage": [...]} report, but got _Map<String, dynamic>.
#0      HitMap.parseFiles (package:coverage/src/hitmap.dart:190:11)
#1      main (.../coverage-1.16.0/bin/format_coverage.dart:74:18)

Repro:

1. A package with one test file tagged @Tags(['skipme']) and tags: {skipme: {skip: true}} in dart_test.yaml.
2. dart test --coverage=coverage
3. dart pub global run coverage:format_coverage --lcov --in=coverage --out=coverage/lcov.info

This works with 1.15.1. Either format_coverage could treat {} as an empty report, or dart test could skip writing the file for suites that collected nothing.