fix: allow to filter out all points by calling `filter([])`

[flekschas]

This PR enables filtering out all points by calling scatterplot.filter([]);

Description

What was changed in this pull request?

This PR enables filtering out all points which was previously not possible. To check if any point filter have been applied, this PR also exposes isPointsFiltered via the get() function.

Why is it necessary?

Fixes #122

Checklist

• [x] Provided a concise title as a semantic commit message (e.g. "fix: correctly handle undefined properties")
• [x] Updated CHANGELOG.md
• [x] Added or updated Tests added or updated
• [x] Documentation added or updated in README.md
• [ ] Example(s) added or updated
• [ ] Screenshot, gif, or video attached for visual changes

[flekschas]

@insertmike for awareness. This should fix the issue you reported.