PhantomJS : How to check for null when iterating through scripts?

I am running a PhantomJS script from node, using a child process and execFile() function. I am passing a url from my node server to the script as an argument. Sometimes the url results in a 404, and no script tags are found within the page. This results in the PhantomJS script crashing, and eventually my app. How can I take care of null values when calling document.getElementsByTagName('script')[i].innerHTML in the following code ?

"use strict";

var page = require('webpage').create();
var system = require('system');
var args = system.args;

page.open(args[1], function () {

  var scriptCtnt = page.evaluate(function() {     
    for (var i = 0; i < 20; i++) {    
        var scriptStr = [].map.call(document.getElementsByTagName('script')[i].innerHTML, function(data) {                    
           return data;                                
        }).join('');    
        if (scriptStr.indexOf('some.field') !== -1) {
          return scriptStr;
        } 
      } 
  });

  var str = scriptCtnt;
  str = str.replace('//<![CDATA[','{');
  str = str.replace(/}},/g,'}}');

  var jsonStr = JSON.stringify(JSON.parse(str));

  console.log(jsonStr);

  phantom.exit();
});

N.B : I tried to make a variable and to check for undefined values, but it didn't work. I also can't deal with this issue in my node api, because neither error nor stderr are called.

The node code calling the PhantomJS script :

      const execFile = require('child_process').execFile;
      let bin = "phantomjs" 
      let args = ['./my.service.js', urlToScrape];
      let jsonStr;

      execFile(bin, args , (error, stdout, stderr) => {

        if (error) {              
          return;
        }

        console.log(`stdout: ${stdout}`);

        if (stdout) {
          res.json(JSON.parse(stdout));              
        } else {
          res.json(stderr);
        }

      }); 

via Stanislasdrg