Clarify a NULL comparison
Clarify a NULL comparison

'TreeItem::get_children()' does not return the child count, but rather
a pointer to the children.

This comparison caused an error during WebAssembly builds using the
LLVM backend path.

(cherry picked from commit 31f929caa2bc8422c0a6bb50f81500e1f4db101b)

pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1