Cleanup.

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

@@ -123,6 +123,7 @@ public class Distance {
         return Objects.hash(distance_m);
     }
 
+    @NonNull
     @Override
     public String toString() {
         return "Distance{" +

src/main/java/de/dennisguse/opentracks/content/data/Speed.java

@@ -1,5 +1,7 @@
 package de.dennisguse.opentracks.content.data;
 
+import androidx.annotation.NonNull;
+
 import java.time.Duration;
 import java.util.Objects;
 
@@ -132,6 +134,7 @@ public class Speed {
         return Objects.hash(speed_mps);
     }
 
+    @NonNull
     @Override
     public String toString() {
         return "Speed{" +

src/main/java/de/dennisguse/opentracks/content/sensor/SensorDataCycling.java

@@ -191,6 +191,7 @@ public final class SensorDataCycling {
                 return speed;
             }
 
+            @NonNull
             @Override
             public String toString() {
                 return "Data{" +

src/main/java/de/dennisguse/opentracks/content/sensor/SensorDataRunning.java

@@ -1,5 +1,6 @@
 package de.dennisguse.opentracks.content.sensor;
 
+import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.VisibleForTesting;
 
@@ -92,10 +93,12 @@ public final class SensorDataRunning extends SensorData<SensorDataRunning.Data>
             return cadence;
         }
 
+        @Nullable
         public Distance getDistance() {
             return distance;
         }
 
+        @NonNull
         @Override
         public String toString() {
             return "Data{" +