[RUM-13793] make resource.url optional

[BenoitZugmeyer]

On some conditions, the Browser SDK sends resource events without URL:

      DD_RUM.init({
        clientToken: 'xxx',
        applicationId: 'xxx',
        beforeSend() {},
      })
      fetch('/' + 'a'.repeat(300_000))

We'll work on a fix, but unfortunately old versions will still be around.

[0xnm]

Since resource.url is nullable, this change will trigger some code changes for languages where nullable type is different from non-nullable.

Will it be a permanent change? Was sending empty string or maybe not sending Resource event at all considered?

[BenoitZugmeyer]

Let's discuss internally

[BenoitZugmeyer]

Resources without URLs will be dropped by the backend.