roachtest: guard against nil log file reference

[renatolabs]

These checks prevent panics in the roachtes test runner, which could jeopardize an entire test run; if the checks fail, we might not create ".failed" files for commands that fail.

Example panic:

https://teamcity.cockroachdb.com/viewLog.html?tab=buildLog&logTab=tree&filter=debug&expand=all&buildId=12162051&_focus=7888

Epic: none

Release note: None

[cockroach-teamcity]

This change is 

[renatolabs]

Similar changes have been made in the past: #93124. I don't completely understand the exact circumstances that cause the (*clusterImpl).l instance to be missing a file handle. Ideally, we would fix that bug instead. In practice, figuring out the reason and making sure we apply the correct change is going to be tricky given the state of test_runner so, given the low stakes at play (missing .failed file), this quick fix is probably a good trade-off.

[srosenberg]

I seem to recall chasing the root cause for these but not succeeding... let's leave it as an exercise for our next intern; kidding. The workaround is sensible, eventually we'll find the culprit.

[renatolabs]

TFTR!

bors r=srosenberg

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)