When I check if an empty string is in an array, POSTMAN always returns true

I am using the pre-script method to loop through test data. I am also setting a validation set of data in the prescript file.

// fetching the test data
var usernames = pm.environment.get("usernames");
var valid_usernames = pm.environment.get("valid_usernames");

// Setting the test data
    usernames = ['ww12345',' ',123,'www.google.com'];

// Setting the verification data
    valid_usernames = ['ww12345','testing'];

// Reseting the loop counter for clicking the 'send' button in top right corner
if(usernames.length === 0){
    usernames = ['ww12345','',123,'www.google.com'];

// looping through the test data
var currentUsername = usernames.shift();
pm.environment.set("username", currentUsername);
pm.environment.set("usernames", usernames);

Then during the test execution, I am calling the current username, checking if it exists in the validation data, and then checking if the response was 200 or 400.

// Checking status codes for valid and invalid users
var valid_usernames = pm.environment.get("valid_usernames");
var username = pm.environment.get("username");
pm.test("Status Code for User", function () {
        console.log("Status code 200 for : " + username);
        console.log("Status code 400 for : " + username);

The only issue is that I am testing if an empty string returns a 400. However, I do not put an empty string into my validation set, but it returns true as if I did. I’m pretty sure this is because Postman is treating my validation set as a string, rather than an array. And a string always contains an empty string, but an array does not always contain an empty string. You can check for yourself here:

I believe the behavior is pretty inconsistent. For looping over my data set, Postman operates as if it is an array, but for the validation set, Postman treats it as a string. From what I found, I cannot ‘typecast’ a variable as an array.
I am looking for a solution that can let me treat valid_usernames as an array rather than working with it as a string
I have the following:
if(username && valid_usernames.includes(username)){}

but in the case that i need to check the empty string, then it will be an extra if statement, and I’m trying to stay DRY. Thanks

I also just tried with no success:

in Pre-request Script (after deleting variable manually from environment):

var valid_usernames = ['qx18264',456];
pm.environment.set("valid_usernames", JSON.stringify(valid_usernames));

and then in Tests:

var valid_usernames = JSON.parse(pm.environment.get("valid_usernames"));

for reference, in environments, the variable is set to:

by no sucess I mean that the following if statement returns true when username===’ ', even though I expect it to return false: