[MAP] Adds fallback to original value when input not found (#13560)

* [MAP] Adding fallback to original value
Solves #10092

Signed-off-by: clinique <gael@lhopital.org>

bundles/org.openhab.transform.map/src/main/java/org/openhab/transform/map/internal/MapTransformationService.java

@@ -52,12 +52,12 @@ import org.slf4j.LoggerFactory;
         "openhab.transform=MAP" })
 public class MapTransformationService
         implements TransformationService, ConfigOptionProvider, RegistryChangeListener<Transformation> {
-    private final Logger logger = LoggerFactory.getLogger(MapTransformationService.class);
-
+    private static final String SOURCE_VALUE = "_source_";
     private static final String PROFILE_CONFIG_URI = "profile:transform:MAP";
     private static final String CONFIG_PARAM_FUNCTION = "function";
     private static final Set<String> SUPPORTED_CONFIGURATION_TYPES = Set.of("map");
 
+    private final Logger logger = LoggerFactory.getLogger(MapTransformationService.class);
     private final TransformationRegistry transformationRegistry;
     private final Map<String, Properties> cachedTransformations = new ConcurrentHashMap<>();
 
@@ -89,6 +89,8 @@ public class MapTransformationService
                     target = properties.getProperty("");
                     if (target == null) {
                         throw new TransformationException("Target value not found in map for '" + source + "'");
+                    } else if (SOURCE_VALUE.equals(target)) {
+                        target = source;
                     }
                 }