Do not create info object if not needed

@@ -28,6 +28,12 @@ class Document {
       String keywords,
       String producer})
       : document = PdfDocument(pageMode: pageMode, deflate: deflate) {
+    if (title != null ||
+        author != null ||
+        creator != null ||
+        subject != null ||
+        keywords != null ||
+        producer != null) {
       document.info = PdfInfo(document,
           title: title,
           author: author,
@@ -36,6 +42,7 @@ class Document {
           keywords: keywords,
           producer: producer);
     }
+  }
 
   static bool debug = false;