Fixes a crash caused by NaNs in CombinedChartView with empty data set

[natethegreat44]

Fixes a crash caused by NaNs in CombinedChartView when an empty dataset is passed in.

Also fixes a simlar NaN-related crash when such a chart is touched.

A minimal view controller to reproduce is:

import UIKit
import Charts

class InformationViewController: UIViewController {
    @IBOutlet weak var chartView: CombinedChartView!

    override func viewDidLoad() {
        let xAxisData = [NSDate]()
        let bar1Data = BarChartDataSet(yVals: [BarChartDataEntry]())
        let bar2Data = BarChartDataSet(yVals: [BarChartDataEntry]())

        chartView.xAxis.labelPosition = .Bottom
        chartView.descriptionText = ""
        chartView.animate(xAxisDuration: 0.5, yAxisDuration: 1.0)

        let combinedData = CombinedChartData(xVals: xAxisData)
        combinedData.barData = BarChartData(xVals: xAxisData, dataSets: [bar1Data, bar2Data])

        chartView.data = combinedData
    }
}

[liuxuan30]

hold on, this seems a regression.
Missing data entries should be taken care of before we start rendering. There is a check when setting data:

        set
        {
            if newValue == nil
            {
                print("Charts: data argument is nil on setData()", terminator: "\n")
                return
            }

            _dataNotSet = false
            _offsetsCalculated = false
            _data = newValue

            // calculate how many digits are needed
            calculateFormatter(min: _data.getYMin(), max: _data.getYMax())

            notifyDataSetChanged()
        }

in drawRect, if we detect dataNotSet is true, we won't continue the drawing at all.

However, I checked the commits, @danielgindi delete one condition:

if (newValue == nil || newValue?.yValCount == 0)

for 062a61b Reduce to nil check only (Fixes #417)

@danielgindi, seems we cannot delete newValue?.yValCount == 0? Or we have other code to defend it?

BTW, if it's nan, returning 0 is not a good choice here I think.

[danielgindi]

Maybe keep the older condition - but only print if the newValue is nil? :-)
I think this would satisfy both issues...

[liuxuan30]

I will close this PR and reverse 062a61b in PR #582
@natethegreat44 thanks for pointing out this issue!