vm.getPrimaryPublicIPAddressId() can throw NPE on wedged VM delete cases

I had a wedged VM delete that caused `vm.getPrimaryPublicIPAddressId()` to throw an NPE. Since this is a possible, albeit unlikely scenario, I think `VirtualMachineImpl.getPrimaryPublicIPAddressId()` should check for nulls during this method chain:
```
public String getPrimaryPublicIPAddressId() {
return this.getPrimaryNetworkInterface().primaryIPConfiguration().publicIPAddressId();
}
```

I got the VM into this state by doing a VM delete via the SDK (I believe) while the VM create (also via the SDK) was still in progress. This put the VM in a state where it shows up in the Azure portal Resource Group blade, however when you click on the VM to get more information it shows a "Not found" page. Neither the NIC nor the public IP show up in the RG blade -- essentially the VM "exists" (the SDK thinks the VM is not null), but is in a wedged state and does not have a NIC or a public IP.

**Additional Info**
SDK version 1.5.1

The message from the activity log:
```
"statusMessage": "{\"status\":\"Failed\",\"error\":{\"code\":\"ResourceOperationFailure\",\"message\":\"The resource operation completed with terminal provisioning state 'Failed'.\",\"details\":[{\"code\":\"Conflict\",\"message\":\"The request failed due to conflict with a concurrent request. \"}]}}",
```

And the stack trace:
```
java.lang.NullPointerException: null
at com.microsoft.azure.management.compute.implementation.VirtualMachineImpl.getPrimaryPublicIPAddressId(VirtualMachineImpl.java:1460)
...
```

Start in VirtualMachineImpl.java at getPrimaryPublicIPAddressId(), specifically the chained calls shown in the issue. Reproduce or inspect the wedged VM state where the VM lacks a network interface or public IP, then verify that this method no longer throws an NPE and returns the appropriate result for that state.

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