Fix page labels creation in case they do not yet exist

Author: Snipx

kernel/src/main/java/com/itextpdf/kernel/pdf/PdfCatalog.java

@@ -233,7 +233,7 @@ public PdfNameTree getNameTree(PdfName treeType) {
      * @return
      */
     public PdfNumTree getPageLabelsTree(boolean createIfNotExists) {
-        if (pageLabels == null && createIfNotExists) {
+        if (pageLabels == null && (getPdfObject().containsKey(PdfName.PageLabels) || createIfNotExists)) {
             pageLabels = new PdfNumTree(this, PdfName.PageLabels);
         }
 

kernel/src/main/java/com/itextpdf/kernel/pdf/PdfDocument.java

@@ -1099,7 +1099,7 @@ public void addFileAttachment(String description, PdfFileSpec fs) {
 
     /**
      * This method retrieves the page labels from a document as an array of String objects.
-     * @return
+     * @return {@link String} list of page labels if they were found, or {@code null} otherwise
      */
     public String[] getPageLabels() {
         if (catalog.getPageLabelsTree(false) == null) {