app: fix: do not crash on empty Meta fields

src/ulo-storage-applications/src/main/java/info/mathhub/uloapi/query/Meta.java

@@ -28,15 +28,15 @@ public class Meta {
     public String paratype;
 
     public String getName() {
-        return this.name;
+        return this.nonNullString(this.name);
     }
 
     public String getSourceref() {
-        return this.sourceref;
+        return this.nonNullString(this.sourceref);
     }
 
     public String getParatype() {
-        return this.paratype;
+        return this.nonNullString(this.paratype);
     }
 
     @Override
@@ -47,4 +47,15 @@ public class Meta {
         );
     }
 
+    /**
+     * @return Return argument {@code s} if {@code s} is not {@code null}.
+     * If {@code} s} is {@code null}, return an empty {@code String}.
+     */
+    private String nonNullString(String s) {
+        if (s == null) {
+            return "";
+        } else {
+            return s;
+        }
+    }
 }