Remove warnings related to empty tag-trees.

Remove warnings related to empty tag-trees.

Decoding some valid .jp2 files like Sentinel2 datasets leads to warnings like:
No incltree created.
tgt_create tree->numnodes == 0, no tree created.
No imsbtree created.
tgt_create tree->numnodes == 0, no tree created.

Besides that, the image is correctly decoded. So there is no reason to emit
those warnings.