NPE via boundary condition, document with only XML declaration

[cowtowncoder]

(note: discovered via https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32872 -- root cause FasterXML/woodstox#123)

Looks like there is specific kind of input that may trigger NPE when using Woodstox 6.2.4 or earlier.

[cowtowncoder]

So, the first finding via #461!

Root cause fixed in Woodstox trunk, but will wait for a bit to see what else might be found before releasing Woodstox 6.2.5 (or 6.3.0 if bigger changes needed)