[!!!][BUGFIX] Cast negative int to boolean (true) in BooleanNode

Currently Fluid treats negative integers as false which
results in <f:if condition="-9"> rendering else instead
of then.

When casting integers to boolean using php all integers
other than 0 (zero) are casted to true, just 0 results in
false. Therefore, casting of BooleanNode::convertToBoolean
must be adjusted to act like plain php.

This bugfix is a breaking change as people might have
deliberately used the wrong behaviour.

Releases: 6.2
Resolves: #54243
Resolves: #33597
Change-Id: Iebaa578cf071909610ca0e1abfc9bfaad7459231