Elevation gain: make it optional as Float.

So if no sensor data is provided, it is not stored in the database and not exported.

src/androidTest/java/de/dennisguse/opentracks/services/sensors/ElevationSumManagerTest.java

@@ -24,6 +24,9 @@ public class ElevationSumManagerTest {
 
     @Test
     public void getElevationGainLoss_downhill() {
+        // given
+        elevationSumManager.setConnected(true);
+
         // then
         addSensorValue(elevationSumManager, new float[]{1015f, 1015.01f, 1015.02f, 1015.03f, 1015.04f, 1015.05f, 1015.06f, 1015.07f, 1015.08f, 1015.09f, 1015.10f, 1015.11f, 1015.12f, 1015.13f, 1015.14f, 1015.15f});
 
@@ -31,4 +34,17 @@ public class ElevationSumManagerTest {
         Assert.assertEquals(0f, elevationSumManager.getElevationGain_m(), 0.01);
         Assert.assertEquals(-48.0, elevationSumManager.getElevationLoss_m(), 0.01);
     }
+
+    @Test
+    public void sensorUnavailable() {
+        // given
+        elevationSumManager.setConnected(false);
+
+        // then
+        elevationSumManager.onSensorValueChanged(999f);
+
+        // then
+        Assert.assertNull(elevationSumManager.getElevationGain_m());
+        Assert.assertNull(elevationSumManager.getElevationLoss_m());
+    }
 }