Blender Git Loki

Fix/enhance new RNA path from real ID helpers.

Main issue was that `rna_prepend_real_ID_path()` would return
nothing in case given path was NULL, when it should actually return the
`prefix` computed by `RNA_find_real_ID_and_path()` in that case...

Also make return `real_id` pointer optionnal, no reasons to make it
mandatory here. And some general naming fixes.