Remove unsanitized null from input strings column in rank_tests.cpp

[davidwendt]

Description

Removes a non-empty null entry from a test strings column utility in rank_tests.cpp.
Behavior with unsanitized nulls in at best UB and should not be included in unit tests.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

/merge