Added nullptr check for _FindEntry (like _InsertEntry and related)

[marcodiiga]

Description of Change(s)

Added nullptr check for _FindEntry (_InsertEntry and others all assume that the operation might fail, makes sense to include the _FindEntry as well). This can also happen (albeit erroneously and unsupportedly) in multithreaded situations and possibly in resource constrained conditions.

• I have verified that all unit tests pass with the proposed changes
• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9152

[asluk]

@jesschimein this would be a good one to get in for v24.03 -- let me know if the USD team at Pixar have any particular concerns-- ideally there would have been a unit test case as well, but we weren't able to quite nail down a repro. In any case it's good to guard against crashes here, and I don't think there is any downside to this. Happy to talk through it more though. Thanks!

[pixar-oss]

Thanks for flagging! We're looking at it internally for inclusion for 24.03!