短路问题：user != nil && user.id > book.id && (nil + 1 > 3)

```java
public static void main(String[] args) {
boolean result = (boolean) AviatorEvaluator.execute("user != nil && user.id > book.id && (nil + 1 > 3)");
System.out.println(result);
}
```
希望短路并返回false，但是抛了异常：
```
Exception in thread "main" com.googlecode.aviator.exception.ExpressionRuntimeException: Could not add with
at com.googlecode.aviator.runtime.type.AviatorObject.add(AviatorObject.java:153)
at com.googlecode.aviator.runtime.type.AviatorNil.add(AviatorNil.java:51)
at com.googlecode.aviator.lexer.token.OperatorType.eval(OperatorType.java:108)
at com.googlecode.aviator.runtime.op.OperationRuntime.eval0(OperationRuntime.java:57)
at com.googlecode.aviator.runtime.op.OperationRuntime.eval(OperationRuntime.java:47)
at com.googlecode.aviator.code.OptimizeCodeGenerator.executeOperator(OptimizeCodeGenerator.java:221)
at com.googlecode.aviator.code.OptimizeCodeGenerator.execute(OptimizeCodeGenerator.java:158)
at com.googlecode.aviator.code.OptimizeCodeGenerator.getResult(OptimizeCodeGenerator.java:361)
```

Start by running the expression through AviatorEvaluator.execute and follow the stack from OperatorType.java and OperationRuntime.java into OptimizeCodeGenerator.java. Check how the && expression is evaluated when user is nil. Done means the supplied example returns false without evaluating the invalid nil addition or throwing an exception.

Written by the indexing model from the issue text.