Fix section header type

[smigles]

Section header type changed from string to string | null because "The top is represented as level 1, with header null".

mwn/src/wikitext.ts

Lines 86 to 96 in 4442b4a

	/**
	* Parse sections from wikitext
	* CAUTION: section header syntax in comments, nowiki tags,
	* pre, source or syntaxhighlight tags can lead to wrong results.
	* You're advised to run unbind() first.
	* @returns {Section[]} array of
	* section objects. Each section object has the level, header, index (of beginning) and content.
	* Content *includes* the equal signs and the header.
	* The top is represented as level 1, with header `null`.
	*/
	parseSections(): Section[];