Fix writing explicit XYZ coordinates

[yufeih]

The /XYZ destination needs a zoom value for PDF readers to parse correctly. This PR fixes destination linking problem by adding a NULL zoom value.

Per PDF spec:

[page /XYZ left top zoom] Display the page designated by page, with the coordinates (left, top) positioned at the top-left corner of the window and the contents of the page magnified by the factor zoom. A null value for any of the parameters left, top, or
zoom specifies that the current value of that parameter is to be retained unchanged. A zoom value of 0 has the same meaning as a null value.

#9352