Feature: Nullable values in line chart

[shamilovtim]

This PR provides the ability for a user to provide null points on the line chart to achieve gaps in data.
FlSpot(null,null)

Sample has been updated. It's not pretty but it demonstrates a lot of different stuff.

Closes #26
Closes #123

[shamilovtim]

there are formatting changes again because of the last PR. contributor did not use dart formatter at all

[imaNNeo]

I'll add a linter check to verify it in each PR.
But please revert them, it makes confusion for now and later.

[imaNNeo]

Thank you,
I'm going to the final review.
I think there was an issue for this feature,
could you please find it and mention it in the PR's title?

[shamilovtim]

@imaNNeoFighT Unrelated to this PR but this is what happens on master right now when I provide

       spots: const [
            FlSpot(0, 1),
            FlSpot(11, 4),
            FlSpot(2, 1),
            FlSpot(4.9, 5),
            FlSpot(6.8, 5),
            FlSpot(8, 1),
            FlSpot(9.5, 2),
          ],

Should the spots be sorted or would that be incorrect for a line chart? It seems to be the x or y should determine the order, not the sequence of the elements in the array. Of course, we can always fix it in a different PR. I just wanted to call it out.

[shamilovtim]

Ok all set. Just ignore my last comment, it's unrelated.

[imaNNeo]

About your last comment, I think it's correct :)
because you provided unordered spots, btw you can open an issue to discuss it there.
and about the PR, everything is ready to merge, but we should update the LineChartBarData.spots documentation, (dart docs, above the field and constructor), I will handle it,
Thanks!

[imaNNeo]

I've made some changes on your codes, they are for simplification check the commit here.