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/AltitudeRingBuffer.java

@@ -0,0 +1,32 @@
+package de.dennisguse.opentracks.stats;
+
+import androidx.annotation.Nullable;
+
+import de.dennisguse.opentracks.content.data.Altitude;
+
+public class AltitudeRingBuffer extends RingBuffer<Altitude> {
+
+    private Altitude firstAltitude;
+
+    AltitudeRingBuffer(int size) {
+        super(size);
+    }
+
+    AltitudeRingBuffer(RingBuffer<Altitude> toCopy) {
+        super(toCopy);
+    }
+
+    @Nullable
+    @Override
+    protected Number from(Altitude object) {
+        if (firstAltitude == null) {
+            firstAltitude = object;
+        }
+        return object.toM();
+    }
+
+    @Override
+    protected Altitude to(double object) {
+        return firstAltitude.replace(object);
+    }
+}