fix: treat numeric slider answers as non-empty

[laribright]

Problem

Conditional logic treats every numeric slider answer as empty because Object.keys(number) returns an empty array.

This incorrectly hides questions and sections using notEmpty, and shows those using empty, even when a slider has a valid answer.

Fix

Treat numeric answers, including zero, as non-empty. This matches the existing answer-validation behavior.

Tests

Added regression cases for slider values 0, 5, and 100 covering both empty and notEmpty.