test_runner: fix junit report on empty diagnostic #11100

Job	Run time
Build slim tarball	3s
${{ matrix.system }}: with shared libraries${{ ((matrix.perfetto && ' and perfetto') \|\| '') }}	0s
aarch64-linux: Cache V8 build	1s
aarch64-linux: with shared ${{ matrix.openssl.name }}	0s
	4s

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