Allow Constant Throughput Controller to be disabled by specifying negative value

**olivernz** ([Bug 59448](https://bz.apache.org/bugzilla//show_bug.cgi?id=59448&redirect=false)):
In order to be able to programatically (i.e. through a variable) disable a Constant Throughput Timer it should be possible to enter a negative taget throughput, which would switch off the functionality of this element. This is congruent with the behaviour of Loop Count and Duration in the Thread Group.

OS: All