[Fix] Add parsed serilization when save settings

When loading from existing files or libraries, the parser will set
the serilization of the string constant to the correct value. However,
when editing via the GUI, the serilization was not set and a new
string constant list will be created without the serilization.
This result in the serilization being null and when copying with
the clipboard.

Link to DD2480-Group1#13

src/main/java/org/jabref/gui/libraryproperties/constants/ConstantsPropertiesViewModel.java

@@ -1,6 +1,7 @@
 package org.jabref.gui.libraryproperties.constants;
 
 import java.util.Comparator;
+import java.util.List;
 import java.util.Locale;
 import java.util.Optional;
 import java.util.stream.Collectors;
@@ -86,9 +87,12 @@ private ConstantsItemModel convertFromBibTexString(BibtexString bibtexString) {
 
     @Override
     public void storeSettings() {
-        databaseContext.getDatabase().setStrings(stringsListProperty.stream()
-                                                                    .map(this::fromBibtexStringViewModel)
-                                                                    .collect(Collectors.toList()));
+        List<BibtexString> strings = stringsListProperty.stream()
+                .map(this::fromBibtexStringViewModel)
+                .toList();
+        strings.forEach(string -> string.setParsedSerialization("@String{" +
+                string.getName() + " = " + string.getContent() + "}\n"));
+        databaseContext.getDatabase().setStrings(strings);
     }
 
     private BibtexString fromBibtexStringViewModel(ConstantsItemModel viewModel) {