For #3917. make init(label: String?) convenient initializer (#3973)

* fix #3917. make `init(label: String?)` to be a convenient init to enable auto inheritance.

* add UT for default dataSet label

Source/Charts/Data/Implementations/Standard/ChartDataSet.swift

@@ -31,11 +31,9 @@ open class ChartDataSet: ChartBaseDataSet
         super.init()
     }
     
-    public override init(label: String?)
+    public override convenience init(label: String?)
     {
-        entries = []
-
-        super.init(label: label)
+        self.init(entries: nil, label: label)
     }
     
     @objc public init(entries: [ChartDataEntry]?, label: String?)

Tests/Charts/BarChartTests.swift

@@ -109,6 +109,16 @@ class BarChartTests: FBSnapshotTestCase
         let dataSet = setupDefaultDataSet(chartDataEntries: dataEntries)
         let chart = setupDefaultChart(dataSets: [dataSet])
         ChartsSnapshotVerifyView(chart, identifier: Snapshot.identifier(UIScreen.main.bounds.size), overallTolerance: Snapshot.tolerance)
+
+    }
+
+    func testDefaultBarDataSetLabels()
+    {
+        let dataEntries = setupDefaultValuesDataEntries()
+        let dataSet = BarChartDataSet(entries: dataEntries)
+        dataSet.drawIconsEnabled = false
+        let chart = setupDefaultChart(dataSets: [dataSet])
+        ChartsSnapshotVerifyView(chart, identifier: Snapshot.identifier(UIScreen.main.bounds.size), overallTolerance: Snapshot.tolerance)
     }
 
     func testZeroValues()