Use a numerical constant instead of null check

Bug fix for the following issue with sorting null data plotly#4784

last one I swear

Bug fix for the following:

• touch-dragging the legend: going in for a second touch to continue scrolling no longer 'jumps' the legend.

Fix Branch for the plotly PR is fix3801-legend-touchscrolling: https://github.com/sightmachine/plotly.js/tree/fix3801-legend-touchscrolling

This version, is the above, but branched to legend-mobile-scroll-build,

• then did a 'npm run build',
• then committed the resulting built files to this branch
• then made this version here.

TLDR: this is the version to use for NPM

    "plotly.js": "sightmachine/plotly.js#v1.47.4legend",