`[[]]` and `[[|]]` should not be treated as wikilinks

[BrandonXLF]

The strings "[[]]" and "[[|]]" (empty "wikilinks") are not treated as wikilinks by MediaWiki and shouldn't be treated as such here either.

[lahwaacz]

mwparserfromhell cannot always check if the tokens inside double brackets are empty or |. Consider e.g. [[{{echo|}}]] and [[{{!}}]], these are context-sensitive and mwparserfromhell has no chance to behave the same as MediaWiki because it does not expand templates.