fix: only add entity types which are not null AND not empty

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: Pfeil

src/main/java/edu/kit/datamanager/ro_crate/entities/AbstractEntity.java

@@ -443,7 +443,7 @@ public T addType(String type) {
             if (this.types == null) {
                 this.types = new HashSet<>();
             }
-            if (type != null || !type.isEmpty()) {
+            if (type != null && !type.isEmpty()) {
                 this.types.add(type);
             }
             return self();