[FIX] Do not crash on namespaced validators in argument blocks

[ehennestad]

Summary

A function argument validated by a namespaced function with two or more name parts and no size/class specifier crashes the build:

function out = f(a)
    arguments
        a {x.y.z}
    end
end

File "sphinxcontrib/mat_tree_sitter_parser.py", line 375, in _parse_argument_section
    for arg in arguments:
TypeError: 'NoneType' object is not iterable

The grammar wraps such an argument in an ERROR node, so the argument-block query produces no (property) captures and args is None.

Change

Guard against the None capture in _parse_argument_section and skip the block, mirroring the existing guard in _parse_property_section. The build completes; the unparseable argument simply contributes no metadata (which is not rendered in function output anyway).

Adds a regression test (f_with_namespaced_validator_argument) that crashes without the guard.

Scope

This is a robustness fix — it stops the crash. Correctly parsing these validators requires a fix in the tree-sitter-matlab grammar itself, so this only refs (does not close) the issue.

Refs #335