Fix misaligned slider marks with empty labels

[terminalchai]

Summary

• add an explicit height: 12px to .dash-slider-mark so empty labels keep the same mark box height as labeled marks
• add an integration regression test covering the empty-string marks case from #3715
• add the corresponding unreleased changelog entry

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • add the slider mark height fix
  • add a regression test for empty mark labels
  • add the changelog entry
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

Fixes #3715

[AnnMarieW]

Thanks! Looks good.

[AnnMarieW]

Just needs a changelog entry

[terminalchai]

Added the unreleased changelog entry and updated the PR checklist accordingly.

[KoolADE85]

Thanks for the fix, it's looking good to me.