Fix conditions in #showEm()

* return when the pOriginal is null, not when it is non-null
  (Eclipse noticed a definitive NPE)
* return when running in headless mode (unit tests via maven)
Avoid NPE when the expected tree is indeed null
Remove unneeded @SuppressWarnings("unchecked")
Code clean-up after merge + Rewrote test to use JUnit 4.
PICTImageReaderSpi.canDecodeInput() resets stream twice but marks only once

PICTImageReaderSpi.canDecodeInput() potentially changes the position in
the stream. The initial mark has a reset in the finally block. Fine. But
there is no new mark between reset and PICTImageReader.skipNullHeader.