run-tests: don't append rtendtracing logs if pipe is /dev/null
If HGCATAPULTSERVERPIPE is not set, we set it to /dev/null, so we need to make
sure the value is not /dev/null before appending logs.
This fixes some of test-run-tests.t failures.
Differential Revision: https://phab.mercurial-scm.org/D4401
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