Fix address prefix issue

[AUsfy]

Use AddressPrefixes field instead of AddressPrefix which could be null
Issue #8

[AUsfy]

Adding some documentation about the issue & fix :

• Issue Description:
  The code is failing for new VNets and returns an 500 error.

• Observation:
  I think there has been a recent change in the data representation for subnets. Previously, the field AddressPrefix contained the address prefix of the subnet. Now, this field is empty and the data is found in another field, AddressPrefixes which is an array. We can see it also in the subnet ARM representation in Azure:

• Proposed Solution:
  Update the code to use both AddressPrefix and AddressPrefixes fields to ensure compatibility with both old and new VNets/Subnets.