Bugfix: span gaps over null values beyond scale limits

[marisst]

Currently this configuration

{
    type: 'line',
    data: {
        labels: [0, 10, 20, 30, 40, 50],
        datasets: [{
            data: [3, null, 2, 3, null, 1.5],
            spanGaps: true,
            tension: 0.4
        }]
    },
    options: {
        scales: {
            x: {
                type: 'linear',
                min: 11,
                max: 40,
            }
        }
    }
}

produces

Bug: The line is not spanned over to the points [0, 3] and [50, 1.5] despite spanGaps being true. This happens when the first point beyond x min or x max limits contains a null value ([10, null] and [40, null]).

This PR contains changes in _getStartAndCountOfVisiblePoints in helpers.extras.ts fixing the error. It also contains 3 new tests in controller.line.tests.js.

Now the above configuration results in:

Issues fixed:

• spanGaps not working near min and max limits #11934
• LineChart disappears while zooming in if spanGaps: true and non-null datapoints are out of bounds #11956

[marisst]

@LeeLenaleee Could we add this to version 4.4.8 too? :)