[test-runner] Empty browser list crashes with unfriendly message

[aomarks]

Extremely low priority, just noticed that setting browsers to an empty array in the config file shows this slightly unfriendly error:

(node:34995) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'length' of undefined
    at Object.getTestProgressReport (/Users/aomarks/code/lit-html/packages/tests/node_modules/@web/test-runner-cli/dist/reporter/getTestProgress.js:75:73)
    at Object.getTestProgress (/Users/aomarks/code/lit-html/packages/tests/node_modules/@web/test-runner-cli/dist/reporter/defaultReporter.js:49:38)
    at TestRunnerCli.reportTestProgress (/Users/aomarks/code/lit-html/packages/tests/node_modules/@web/test-runner-cli/dist/cli/TestRunnerCli.js:265:100)
    at TestRunnerCli.start (/Users/aomarks/code/lit-html/packages/tests/node_modules/@web/test-runner-cli/dist/cli/TestRunnerCli.js:65:14)
    at Object.startTestRunner (/Users/aomarks/code/lit-html/packages/tests/node_modules/@web/test-runner-cli/dist/startTestRunner.js:38:9)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:34995) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:34995) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.```