r/storage_account: handling when network_rules is empty

[tombuildsstuff]

This PR supersedes the work done by @samart in #4966 - by instead confirming that the top-level props.NetworkRuleSet object is nil (and returning a default) rather than if the nested elements inside of it are nil

I've also added a couple of nil-checks to prevent some crashes that I spotted whilst passing through

[tombuildsstuff]

Tests pass:

[katbyte]

LGTM 👍

[ghost]

This has been released in version 1.40.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 1.40.0"
}
# ... other configuration ...

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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