[Dexter] Make lldb-dap _post_step_hook more stable  (#157663)

Buildbot cross-project-tests-sie-ubuntu has been unstable recently
(https://lab.llvm.org/buildbot/#/builders/181).

Dexter uses lldb-dap in these tests. Occasionally a one will fail with a
KeyError because of a missing "stackFrames" "source" "path".

I can't reproduce the failure locally, but with PRs #157130 and
#157145 I've got DAP logs from a pass and fail.

In a failure, "path" is missing for a step out of main (off the final brace),
and the passing test has one additional "module" event for libc.so.6.

```
<- {
  "body": {
    "module": {
      "addressRange": "0x7ffff7dd1000",
      "debugInfoSize": "4.9MB",
      "id": "5792732F-7831-58C6-6FB4-F3756458CA24-E46E827D",
      "name": "libc.so.6",
      "path": "/lib/x86_64-linux-gnu/libc.so.6",
      "symbolFilePath": "/usr/lib/debug/.build-id/57/92732f783158c66fb4f3756458ca24e46e827d.debug",
      "symbolStatus": "Symbols loaded."
    },
    "reason": "new"
  },
  "event": "module",
  "seq": 0,
  "type": "event"
}
```

That explains why we get a step that is missing a "path" component in the
failure. I don't understand why LLDB (or LLDB-DAP) is sometimes unable to load
the module (in time?). But "path" is an optional field anyway, so I think it's
worth handling in dexter even if LLDB's behaviour here is confusing.

This commit should stabilize the bot if the only time a module goes missing is
for steps outside main. (Ideally none would go missing, but those shouldn't
interfere with the tests).