TST: Improve branch length testing

[rmcar17]

Root doesn't have a branch length so avoid also check the value actually exists

[coveralls]

Pull Request Test Coverage Report for Build 16639369154

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.572%

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Totals
Change from base Build 16615334304:	0.0%
Covered Lines:	1862
Relevant Lines:	1870

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💛 - Coveralls