[debugger] handle paused state when there are 0 frames

Discussion started here: https://github.com/flutter/devtools/pull/2683#discussion_r572410011

We can get into a state where the app is paused but there are not frames. The original discussion started as a question of whether or not we should support stepping in this case. This question still stands, but sparks a larger question of how we can make this case actionable for our users? If there are neither scripts nor frames to show, what will the user experience be like as the user steps / resumes? If they did not manually pause the app, how will the user understand why the app is paused?

In the web case, the app can be paused in some JS or SDK code for which we don't have any frames (do we have scripts at this point @grouma?). Perhaps we could add some fake frame `JS` that at least communicates to the user that their app is paused in JS code, and then point them to Chrome DevTools for a better JS debugging experience.

@grouma @bkonyi for input on what the debugging workflow should look like in this case for both web and mobile connections.

先阅读 PR #2683 中的讨论，然后追踪 DevTools 在没有可用 frame 或 script 时如何表示暂停状态。关于单步执行、恢复，以及解释 Web 暂停与移动端暂停之间差异的预期行为仍需达成一致；完成意味着实现一个已经确定且可执行的用户体验，并由相关测试覆盖。

由索引模型根据 Issue 内容生成。