Elevation gain: make it optional as Float - and if null, then the UI just shows "-".

In TrackStatistics the elevation gain is now nullable.

src/androidTest/java/de/dennisguse/opentracks/content/provider/CustomContentProviderUtilsTest.java

@@ -455,7 +455,7 @@ public class CustomContentProviderUtilsTest {
         statistics.setTotalTime(1500L);
         statistics.setMovingTime(700L);
         statistics.setTotalDistance(750.0);
-        statistics.setTotalElevationGain(50.0);
+        statistics.setTotalElevationGain(50.0f);
         statistics.setMaxSpeed(60.0);
         statistics.setMaxElevation(1250.0);
         statistics.setMinElevation(1200.0);

src/androidTest/java/de/dennisguse/opentracks/stats/TrackStatisticsTest.java

@@ -51,8 +51,8 @@ public class TrackStatisticsTest {
         statistics2.setMovingTime(600L);  // Result: 700+600
         statistics.setTotalDistance(750.0);
         statistics2.setTotalDistance(350.0);  // Result: 750+350
-        statistics.setTotalElevationGain(50.0);
-        statistics2.setTotalElevationGain(850.0);  // Result: 850+50
+        statistics.setTotalElevationGain(50.0f);
+        statistics2.setTotalElevationGain(850.0f);  // Result: 850+50
         statistics.setMaxSpeed(60.0);  // Resulting max speed
         statistics2.setMaxSpeed(30.0);
         statistics.setMaxElevation(1250.0);

src/androidTest/java/de/dennisguse/opentracks/viewmodels/AggregatedStatisticsTest.java

@@ -35,7 +35,7 @@ public class AggregatedStatisticsTest {
         statistics.setTotalTime(totalTime);
         statistics.setMovingTime(totalTime);
         statistics.setTotalDistance(totalDistance);
-        statistics.setTotalElevationGain(50.0);
+        statistics.setTotalElevationGain(50.0f);
         statistics.setMaxSpeed(50.0);  // Resulting max speed
         statistics.setMaxElevation(1250.0);
         statistics.setMinElevation(1200.0);  // Resulting min elevation