fix: remove null items while authentication is not specified

[TryV]

Exported collection was like this:

...
"header": [
	{
		"key": "Accept",
		"value": "application\/json"
	},
	{
		"key": "Content-Type",
		"value": "application\/json"
	},
	null
],
...

Postman error:

Error while importing Postman Collection v2.1: Cannot read properties of null (reading 'disabled')

[tomirons]

Thank you for your contribution!