Fix #105 detect missing subnet properties

[cchildress]

#105

We should catch missing security group and route table entries for subnets if they're missing from the Azure API response. This sets both to an empty string in the read function if no result was returned from the Azure API.

[tombuildsstuff]

Hi @cchildress

Thanks for this PR - this LGTM and the tests pass:

$ TF_ACC=1 envchain azurerm go test ./azurerm -v -run TestAccAzureRMSubnet -timeout 300m -parallel 5
=== RUN   TestAccAzureRMSubnet_importBasic

--- PASS: TestAccAzureRMSubnet_importBasic (136.29s)
=== RUN   TestAccAzureRMSubnet_importWithRouteTable
--- PASS: TestAccAzureRMSubnet_importWithRouteTable (142.40s)
=== RUN   TestAccAzureRMSubnet_basic
--- PASS: TestAccAzureRMSubnet_basic (144.70s)
=== RUN   TestAccAzureRMSubnet_routeTableUpdate
--- PASS: TestAccAzureRMSubnet_routeTableUpdate (168.59s)
=== RUN   TestAccAzureRMSubnet_disappears
--- PASS: TestAccAzureRMSubnet_disappears (118.39s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	710.395s

Thanks!

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