Migrate KEmptyPlaceholder to use KOptionalText

[AlexVelezLl]

Summary

Migrate KEmptyPlaceholder to use KOptionalText, a new component introduced in the PR: learningequality/kolibri-design-system#403

Reviewer guidance

Link the KDS dependency with the branch of the requested PR. Then, go to any of the sections where KEmptyPlaceholder is used, and KOptionalText should replay.

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0a7.dev0_git.65.g390c74fd.pex
Windows Installer (EXE)	kolibri-0.16.0a7.dev0+git.65.g390c74fd-unsigned.exe
Debian Package	kolibri_0.16.0a7.dev0+git.65.g390c74fd-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0a7.dev0+git.65.g390c74fd-0.3.0.dmg
Source Tarball	kolibri-0.16.0a7.dev0+git.65.g390c74fd.tar.gz
WHL file	kolibri-0.16.0a7.dev0+git.65.g390c74fd-py2.py3-none-any.whl