Bugfix: return nearest non-null point on interaction when spanGaps=true

[marisst]

Currently this configuration

{
    type: 'line',
    data: {
        labels: [1, 2, 3, 4, 5, 6, 7, 8, 9],
        datasets: [{
            label: "# of Votes",
            data: [12, 19, null, null, null, null, 5, 2, 3],
            spanGaps: true,
        }]
    },
    options: {
        interaction: {
            mode: 'nearest',
            axis: 'x',
            intersect: false
        }
    }
}

produces the following interaction - please pay attention to when and where the tooltip appears.

before.mov

Bug: No tooltip is shown while the mouse is between x: 3 and x: 6 despite spanGaps being true. This happens when chart contains null points.

This PR contains changes in binarySeach wrapper function in core.interaction.js fixing the error. It also contains 1 new test with 12 test cases in core.interaction.tests.js.

Now the above configuration results in:

after.mov

Issues fixed:

• Null points with nearest interaction with spanGaps: true #10325