FIX : undefined references is a valid use case.

I tried to mitigate the breaking change by keep on throwing in most uses of the change method.

src/UglyToad.PdfPig/Tokenization/Scanner/PdfTokenScanner.cs

@@ -664,7 +664,7 @@
 
             if (!objectLocationProvider.TryGetOffset(reference, out var offset))
             {
-                throw new InvalidOperationException($"Could not find the object with reference: {reference}.");
+                return null;
             }
 
             // Negative offsets refer to a stream with that number.