fix label generation if canvas generated on scroll

[jpsimard-nyx]

When using another plugin (chartjs-plugin-deferred) the script was failing with :

chartjs-plugin-labels.js:84 Uncaught TypeError: Cannot read property 'meta' of undefined
at Label../node_modules/chartjs-plugin-labels/src/chartjs-plugin-labels.js.Label.renderToDataset (chartjs-plugin-labels.js:84)
at Array.forEach ()
at Label../node_modules/chartjs-plugin-labels/src/chartjs-plugin-labels.js.Label.render (chartjs-plugin-labels.js:77)
at chartjs-plugin-labels.js:486
at Array.forEach ()
at Object.afterDatasetsDraw (chartjs-plugin-labels.js:485)
at Object.notify (Chart.js:7173)
at Chart.drawDatasets (Chart.js:8948)
at Chart.draw (Chart.js:8908)
at Chart.render (Chart.js:8866)
at Object.callback (Chart.js:1778)
at Object.advance (Chart.js:2952)
at Object.startDigest (Chart.js:2925)
at Chart.js:2914

It is basically because the canvas is rendered but chart not drawned yet. This modification testing arg for null/undefined/false fixes the problem.