test: Check for some unlikely null dereferences in tests

[ximinez]

High Level Overview of Change

I stumbled upon these couple of places where test code could, but is very unlikely to, dereference a null pointer. I added a simple check to each of them.

Context of Change

This was a random find.

Even though these changes are tiny, I don't think they are insignificant enough to be considered "trivial", so I'm requesting two reviews.

Type of Change

• [X] Bug fix (non-breaking change which fixes an issue)
• [X] Tests (you added tests for code that already exists, or your new feature included in this PR)

Before / After

There should be no observable change, as tests aren't currently likely / able to get a null value.

Future Tasks

Maybe there are more of these?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.9%. Comparing base (5aa1106) to head (540713d).

Additional details and impacted files

[](https://app.codecov.io/gh/XRPLF/rippled/pull/5004?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) ```diff @@ Coverage Diff @@ ## develop #5004 +/- ## ========================================= - Coverage 70.9% 70.9% -0.0% ========================================= Files 796 796 Lines 66792 66792 Branches 11002 11000 -2 ========================================= - Hits 47379 47372 -7 - Misses 19413 19420 +7 ``` [see 3 files with indirect coverage changes](https://app.codecov.io/gh/XRPLF/rippled/pull/5004/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) [](https://app.codecov.io/gh/XRPLF/rippled/pull/5004?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF)