report(treemap): correct percentages when 0 bytes JS

[brendankenny]

when there were 0 total bytes JS on a page, treemap will take percentages of that 0 and happily show you

https://example.com/ · Resource Bytes: 0 B (NaN%)

We saw it on example.com, but also noticed by at least one user.

The "correct" percentage is arguable but since there's just the one entry on the page 100% seemed more reasonable than 0%, since that just leaves you wondering where the rest of the 0 bytes are :)

Testing is awkward. I could change debug.json to include a subnode with 0 total but it seems fiddly (would the test have to click into the subtree to trigger this path?). Add a new pptr test just for this?

fixes #13383

[connorjclark]

Am fine with no tests, but if that makes you queezy it'd be best to make unit tests for makeCaption rather than an integration test.

[brendankenny]

Am fine with no tests, but if that makes you queezy it'd be best to make unit tests for makeCaption rather than an integration test.

yeah, it's fine. A possible regression here isn't fatal or that confusing (NaN is arguably a correct percentage too :), and probably best for someone else to think about ways to increase test coverage for treemap