Fix null object conversion error in AST node

[b1rdfree]

In escodegen.js, converting a null object into source code in AST will generate an error on toString(). This is because the original judgment considers that the null object node has a "value" key value. Previous code：escodegen.js-----Lines 2310 to 2313： if (expr.value === null) {return 'null'; } I think I can change the above code to: if (expr.value === null || (expr.raw === 'null' && expr.value===undefined)) {return 'null'; } This way it can be converted from the ast tree to the original code when it encounters a null object normally. testcase code(Nodejs environment): var esprima=require("esprima"); var escodegen=require("escodegen"); var ast=esprima.parse("var a=null;"); var code=escodegen.generate(ast);