Fix null script_name field in worker script settings

[gnharishkumar13]

Related to #1517

Allow the script_name field in the worker script's settings to be null.

• Change the ScriptName field in WorkerDurableObjectBinding struct to a pointer to a string in workers_bindings.go.
• Update the serialize method in WorkerDurableObjectBinding to handle null values for script_name.
• Update the ListWorkerBindings function in workers_bindings.go to handle null script_name values.
• Add test cases in workers_bindings_test.go to verify handling of null script_name values.
• Update the formatMultipartBody function in workers.go to handle null script_name values.
• Add test cases in workers_test.go to verify handling of null script_name values.

[github-actions]

Oops! It looks like no changelog entry is attached to this PR. Please include a release note as described in https://github.com/cloudflare/cloudflare-go/blob/master/docs/changelog-process.md.

Example:

```release-note:TYPE
Release note
```

If you do not require a release note to be included, please add the workflow/skip-changelog-entry label.