Filter marker schema with an empty "graphs" array out of the custom tracks area

[julienw]

With this PR, when the local tracks are computed, marker schemas with a graphs property that is an empty array are silently skipped. Previously only schemas without a graphs property were skipped.

This should fix the crash in #4818, but the script that generates the data should be fixed as well.

I added a test as the first commit so that we can see the change in the second commit.

[codecov[bot]]

Codecov Report

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

Project coverage is 88.49%. Comparing base (a43a86c) to head (c0c32c3).

:exclamation: Current head c0c32c3 differs from pull request most recent head 9b40024

Please upload reports for the commit 9b40024 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5020 +/- ## ======================================= Coverage 88.49% 88.49% ======================================= Files 304 304 Lines 27398 27400 +2 Branches 7402 7404 +2 ======================================= + Hits 24245 24247 +2 Misses 2931 2931 Partials 222 222 ```

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