Crash report generated on "Export Selected" when selected track is muted

**Describe the bug**
Using "Export Selected Audio" triggers a crash report if audio is muted

**To Reproduce**
Steps to reproduce the behavior:
1. Select a muted track
2. "File menu > Export > Export Selected Audio"
3. Observe crash report:
`{
"timestamp": 1637352350,
"event_id": "91e58e3fad2b431f8ea7b90bff2d50d9",
"platform": "native",
"release": "audacity@3.1.2",
"contexts": {
"os": {
"type": "os",
"name": "Unix",
"version": "5.11.0"
}
},
"exception": {
"values": [
{
"type": "Warning",
"value": "All selected audio is muted.",
"mechanism": {
"type": "runtime_error",
"handled": false
}
}
]
}
}`

**Expected behavior**
A warning about the muted audio, but no crash report

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Additional information (please complete the following information):**
- OS: Linux and Windows
- Version [e.g. Audacity 3.1.2]

**Additional context**
Add any other context about the problem here.

Start by reproducing the issue with a muted track through File > Export > Export Selected Audio and inspect the export path that handles all-muted selections. The fix is complete when this case shows a warning about muted audio without generating a crash report, while normal exports remain unaffected.

Written by the indexing model from the issue text.