Do not show enum metadata which is not set

The exercise metadata component is only rendered
when it gets a value which is not null and has
a length of at least one (either string or array).
If it gets a value which is translated before it
is being passed, the result will be something of
the form "translation-not-found[...]", which has
a length of at least one, so it is rendered.
To prevent this, check if a value is set before
translating it.

src/main/webapp/app/exercise/exercise-details/exercise-metadata/exercise-metadata.component.html

@@ -75,6 +75,7 @@
     </jhi-exercise-metadata-item>
 
     <jhi-exercise-metadata-item
+        *ngIf="exercise.difficulty"
         [description]="'exercise.metadata.difficulty'"
         [value]="'exercise.metadata.' + exercise.difficulty | translate">
     </jhi-exercise-metadata-item>
@@ -90,16 +91,19 @@
     </jhi-exercise-metadata-item>
 
     <jhi-exercise-metadata-item
+        *ngIf="exercise.type"
         [description]="'exercise.metadata.type'"
         [value]="'exercise.metadata.' + exercise.type | translate">
     </jhi-exercise-metadata-item>
 
     <jhi-exercise-metadata-item
+        *ngIf="exercise.structure"
         [description]="'exercise.metadata.structure'"
         [value]="'exercise.metadata.' + exercise.structure | translate">
     </jhi-exercise-metadata-item>
 
     <jhi-exercise-metadata-item
+        *ngIf="exercise.status"
         [description]="'exercise.metadata.status'"
         [value]="'exercise.metadata.' + exercise.status | translate">
     </jhi-exercise-metadata-item>