TrackStatistics and ChartView: handle altitude and speed as optional with null as default value.

Then only the relevant part of the y-axis is shown.

Fixes #795.

src/main/java/de/dennisguse/opentracks/stats/SpeedRingBuffer.java

@@ -0,0 +1,27 @@
+package de.dennisguse.opentracks.stats;
+
+import androidx.annotation.Nullable;
+
+import de.dennisguse.opentracks.content.data.Speed;
+
+public class SpeedRingBuffer extends RingBuffer<Speed> {
+
+    SpeedRingBuffer(int size) {
+        super(size);
+    }
+
+    SpeedRingBuffer(RingBuffer<Speed> toCopy) {
+        super(toCopy);
+    }
+
+    @Nullable
+    @Override
+    protected Number from(Speed object) {
+        return object.toMPS();
+    }
+
+    @Override
+    protected Speed to(double object) {
+        return Speed.of(object);
+    }
+}