Feature/fix npe in case there is no metadata

pom.xml

@@ -6,7 +6,7 @@
 
     <groupId>de.app.5gla</groupId>
     <artifactId>fiware-integration-layer</artifactId>
-    <version>10.0.0</version>
+    <version>10.1.0</version>
 
     <name>5gLa FIWARE integration layer</name>
     <url>https://github.com/vitrum-connect/5gla-fiware-integration-layer</url>

src/main/java/de/app/fivegla/fiware/model/builder/DeviceMeasurementBuilder.java

@@ -62,22 +62,23 @@ public DeviceMeasurementBuilder withLocation(double latitude, double longitude)
      * @return the DeviceMeasurementBuilder instance with the updated measurement attribute
      */
     public DeviceMeasurementBuilder withMeasurement(String name, String type, String value, Instant dateObserved, MetadataEntry... metadataEntries) {
-        var attribute = new Attribute();
-        attribute.setName(name);
-        attribute.setType(type);
-        attribute.setValue(value);
+        var measurement = new Attribute();
+        measurement.setName(name);
+        measurement.setType(type);
+        measurement.setValue(value);
+        var metadata = new ArrayList<Metadata>();
         var dateObservedMetadata = new Metadata();
         dateObservedMetadata.setName("dateObservedMetadata");
         dateObservedMetadata.setType(FiwareTypes.DATE_TIME.getKey());
         dateObservedMetadata.setValue(dateObservedMetadata.toString());
+        metadata.add(dateObservedMetadata);
         if (null != metadataEntries && metadataEntries.length > 0) {
-            var metadata = new ArrayList<Metadata>();
             for (var metadataEntry : metadataEntries) {
                 metadata.add(metadataEntry.asMetadata());
             }
-            attribute.setMetadata(metadata);
+            measurement.setMetadata(metadata);
         }
-        deviceMeasurement.setMeasurement(attribute);
+        deviceMeasurement.setMeasurement(measurement);
         return this;
     }
 

src/main/java/de/app/fivegla/fiware/model/generic/Attribute.java

@@ -51,6 +51,10 @@ public String asJson() {
     }
 
     private String metadataAsJson() {
-        return metadata.stream().map(Metadata::asJson).collect(Collectors.joining(","));
+        if (metadata == null || metadata.isEmpty()) {
+            return "";
+        } else {
+            return metadata.stream().map(Metadata::asJson).collect(Collectors.joining(","));
+        }
     }
 }