Fix type body parsing w.r.t. optional body types

[amtelekom]

The PatternStmt substatement is so generic, it also matches on other substatements.
Checking if it matches last works around this problem, although there might be a more robust solution.

[sjtakada]

@amtelekom Can you provide any example of pattern substatement which causes the problem?

[amtelekom]

Hi,

I've since come across more problems with parsing type bodies, so more investigation & this more robust fix were needed.

The problem was that many type bodies have only optional parameters. Since collect_vec_stmt can parse any substatements into an empty PatternStmt, because all its substatments are optional,
if let Ok(pattern) = collect_nonempty_vec_stmt!(stmts, PatternStmt) always triggered. This made many types get assigned the wrong type-body.

To fix it, I've introduced a new version of collect_vec_stmt, collect_nonempty_vec_stmt, that will only return Ok if at least one substmt was able to be added to the vec.

I've also added 2 testcases from ietf yang files that fail against current main, but work with this fix.