fix: skip empty TightParagraph without panicking - #1128

Problem

Parsing certain Markdown with Options::ENABLE_TASKLISTS and Options::ENABLE_STRIKETHROUGH could panic while iterating the parser:

called `Option::unwrap()` on a `None` value
  at parse.rs (TightParagraph handling in next_event_range)

Reproducer from #1084:

let markdown_input = "* [ ] ~![=?\\*\x0c\x00\x00  \x0d* [  1=1\x00\x0d<!]:[=?\\\x0d\x0c\n* [ ] \x0d\x0c%    ";
let mut options = Options::empty();
options.insert(Options::ENABLE_TASKLISTS);
options.insert(Options::ENABLE_STRIKETHROUGH);
for _ in Parser::new_ext(markdown_input, options) {}

A related empty-TightParagraph panic was reported in #1095.

Root cause

Tight list items use ItemBody::TightParagraph nodes that intentionally emit neither Start nor End. The iterator always did tree.push() then tree.cur().unwrap() to descend into the paragraph body. When the tight paragraph had no children, cur was None and the unwrap panicked.

Fix

When encountering a TightParagraph:

1. push() into it.
2. If it has children, continue iteration from the first child (same as before).
3. If it is empty, pop(), advance to the next sibling, and recurse into next_event_range so remaining events are still emitted.

This is intentionally different from returning None on an empty tight paragraph, which would end iteration early and drop later siblings or ancestor ends.

Note: a similar fix landed on branch_0.13 via #1096 / #1101, but not on main after the ParserInner refactor. This ports the correct empty-paragraph handling to current main.

Test plan

• cargo test -p pulldown-cmark --lib (includes new issue_1084 and issue_1095)
• cargo test -p pulldown-cmark (full package, 1241+ suite tests)
• Confirmed the Panic (Option::unwrap() on None) in parse.rs:2367 when Tasklists and Strikethrough are enabled #1084 input panics on tag v0.13.3 and does not panic with this change

Fixes #1084