[wasm] Debugger tests build broken on `main`

[radical]

/workspaces/runtime/src/mono/wasm/debugger/DebuggerTestSuite/TestHarnessProxy.cs(135,44): error CS8601: Possible null reference assignment

Started being hit with #82479 . It wasn't caught here because the wasm debugger tests job didn't run.
The other instances of this were fixed in #82803 .

Known Issue Error Message

Fill the error message using known issues guidance.

{
  "ErrorMessage": "TestHarnessProxy.cs(135,44): error CS8601: Possible null reference assignment",
  "BuildRetry": false
}

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Issue Details

---

/workspaces/runtime/src/mono/wasm/debugger/DebuggerTestSuite/TestHarnessProxy.cs(135,44): error CS8601: Possible null reference assignment

Started being hit with #82479 . It wasn't caught here because the wasm debugger tests job didn't run.
The other instances of this were fixed in #82803 .

Author:	radical
Assignees:	-
Labels:	arch-wasm, blocking-clean-ci, area-Infrastructure-mono
Milestone:	-