mishoo/UglifyJS — pull request #408 by RGustBardon (2012-06-05 12:10:30+00:00)
Title: Remove NOT in IfStatement with EmptyStatement (-s)

If the only statement of a nonterminal IfStatement is a nonterminal
EmptyStatement or a nonterminal IfStatement consists of two
nonterminal EmptyStatement symbols, such a statement is transformed
into a nonterminal ExpressionStatement that consists of a
UnaryExpression with the logical NOT operator and the expression found
in the nonterminal IfStatement. The operator in question is
superfluous.

This patch omits the negation by detecting the described case earlier.