Intervals: if gain/loss is not available show "-" instead of null.

Fixes #1338.

src/main/java/de/dennisguse/opentracks/data/models/Distance.java

@@ -13,6 +13,14 @@ public class Distance {
         return new Distance(distance_m);
     }
 
+    public static Distance of(Double distance_m) {
+        if (distance_m == null) {
+            return Distance.of(Double.NaN);
+        } else {
+            return Distance.of((double) distance_m);
+        }
+    }
+
     public static Distance of(String distance_m) {
         return of(Float.parseFloat(distance_m));
     }

src/main/java/de/dennisguse/opentracks/data/models/DistanceFormatter.java

@@ -42,12 +42,13 @@ public class DistanceFormatter {
      */
     public Pair<String, String> getDistanceParts(Distance distance) {
         if (distance.isInvalid()) {
+            String valueUnknown = resources.getString(R.string.value_unknown);
             switch (unitSystem) {
                 case METRIC:
-                    return new Pair<>(null, resources.getString(R.string.unit_meter));
+                    return new Pair<>(valueUnknown, resources.getString(R.string.unit_meter));
                 case IMPERIAL:
                 case NAUTICAL_IMPERIAL:
-                    return new Pair<>(null, resources.getString(R.string.unit_feet));
+                    return new Pair<>(valueUnknown, resources.getString(R.string.unit_feet));
                 default:
                     throw new RuntimeException("Not implemented");
             }