fix(webapp): hide timeline tooltip if there's no data

[eh-am]

Initially raised #1469 (comment)

We always render a tooltip, even if there's no body:

It's a bit tricky to solve this, since it would require the parent to know whether the children is empty. Instead of manually rendering I decided to just use CSS to hide the tooltip if it's empty (ie its children is null).

[github-actions]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
webapp/public/assets/app.js	420.48 KB (0%)	8.5 s (0%)	3.2 s (-15.53% 🔽)	11.7 s
webapp/public/assets/app.css	15.68 KB (+0.04% 🔺)	314 ms (+0.04% 🔺)	0 ms (+100% 🔺)	314 ms
webapp/public/assets/styles.css	9.47 KB (0%)	190 ms (0%)	0 ms (+100% 🔺)	190 ms
packages/pyroscope-flamegraph/dist/index.js	91.8 KB (0%)	1.9 s (0%)	1.6 s (+55.58% 🔺)	3.5 s
packages/pyroscope-flamegraph/dist/index.node.js	91.65 KB (0%)	1.9 s (0%)	635 ms (-13.44% 🔽)	2.5 s
packages/pyroscope-flamegraph/dist/index.css	7.12 KB (0%)	143 ms (0%)	0 ms (+100% 🔺)	143 ms

[codecov]

Codecov Report

Merging #1472 (2d82199) into main (c0d4152) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1472   +/-   ##
=======================================
  Coverage   68.66%   68.66%           
=======================================
  Files         131      131           
  Lines        4316     4316           
  Branches     1166     1166           
=======================================
  Hits         2963     2963           
  Misses       1347     1347           
  Partials        6        6           

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[pavelpashkovsky]

LGTM!