stacktrace frame vars reported as NaN

**Describe the bug**:

As part of the ops KPI review we've been investigating the following errors in ecs logs:

```
decode error: data read error: v2.errorRoot.Error: v2.errorEvent.Exception: v2.errorException.Stacktrace: []v2.stacktraceFrame: v2.stacktraceFrame.Vars: Read: unexpected value type: 0, error found in #10 byte of ...|length": [NaN, NaN, |..., bigger context ...| redacted ...", "common_length": [NaN, NaN, NaN, NaN, NaN, NaN, 27.0, 76.0, 76.0, 76|...
```

`vars` is defined as a flat mapping of local variables but `NaN` is not a valid type.

**To Reproduce**

I don't have steps to reproduce, this is part of the ops KPI review and was observed in the ecs logs.

**Environment (please complete the following information)**

The error has been observed in the following agent versions:
- apm-agent-python/6.18.0
- apm-agent-python/6.15.1
- apm-agent-python/6.13.2
- apm-agent-python/6.12.0
- apm-agent-python/6.10.0
- apm-agent-python/6.7.2
- elasticapm-python/5.10.1
- elasticapm-python/5.10.0

This happens on multiple apm-server versions.

**Additional context**

none