MX::isNull --> MX::isNullE in MXFunctionInternal, #380
MX::isNull --> MX::isNullE in CallFX, #380
MX::isNull --> MX::isNullE in mx_tools, #380
MX::isNull --> MX::isNullE in FXInternal, #380
MX::isNull --> MX::isNullE in XFunctionInternal, #380
Changed default constructor of MX to return a 0-by-0 matrix instead of a null pointer, #380
Removed helper functions for #380
Removed Matrix::isNull, fixed broken unit tests, #380
Simplifications in MXFunctionInternal, post #380
Simplifications in MX, post #380
Simplifications in MXNode, post #380, refactored MXNode destructor