fix: attach doc comments to inner nodes in short-form if/elseif/else

[jorgsowa]

Fixes #1166

1. Swap node("block") before next() — next() consumes : and simultaneously reads any following comments into _docs. By calling node("block") first (before : is eaten), those comments stay in _docs for the first inner node — e.g., a function — to claim as its own leadingComments. This makes the short-form if/elseif/else consistent with read_code_block and read_short_form, which already do node("block") before consuming {.
2. noop injection for empty blocks — when a short-form block has no children but there are floating comments inside (between : and endif), a noop node is injected to absorb them as leadingComments. Without this, the comments had
   nowhere to attach and drifted to trailingComments on an unrelated node. Again, this aligns with read_code_block and read_short_form, which already had this guard.

Let me know if the second change is correct. The logic after the fix was not convincing to me, so I decided to align the behaviour for all blocks without children.

[czosel]

Released in v3.5.0 🎉