Generate bytecode to evaluate the expression leaving a plain Java value on the stack. This method
must only be called if the static type of the expressionis such that the value is known to be representable
by a value of the specified class; the cardinality must be either exactly-one or zero-or-one.
requiredClass - the class of the Java value that is to be left on the stack if the generated code
exits normally. This must be one of Integer.TYPE, Double.TYPE, Float.TYPE, BigDecimal.class,
onEmpty - defines the action to be taken if the expression evaluates to the empty sequence.
May be null if the expression is known statically not to evaluate to an empty sequence. If the return
class is a primitive type (double, float, etc) this must be an instance of OnEmpty.UnwindAndJump