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.

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