Bugfix: NPE in GPXTrackExporter.

Introduced in 0f489f1cc9

Fixes #1176.

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

@@ -1,6 +1,7 @@
 package de.dennisguse.opentracks.data.models;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 
 import java.util.Objects;
 
@@ -14,6 +15,14 @@ public class Distance {
         return of(Float.parseFloat(distance_m));
     }
 
+    @Nullable
+    public static Distance ofOrNull(Double distance_m) {
+        if (distance_m == null) {
+            return null;
+        }
+        return of(distance_m);
+    }
+
     public static Distance ofMile(double distance_mile) {
         return of(distance_mile * UnitConversions.MI_TO_M);
     }