Fix #578: list with checkbox mixed with empty lines

[tomyeh]

taskListItemState shall be reset before calling parseTaskListItem when detecting a list pattern. O/W, it will generate a checkbox for an empty item.

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[coveralls]

Pull Request Test Coverage Report for Build 7972471086

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.135%

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Totals
Change from base Build 7907947836:	0.0%
Covered Lines:	1542
Relevant Lines:	1604

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💛 - Coveralls