provider/azurerm: fix computed tags causing non-empty plan

The tag schema was changed in hashicorp#9199, setting the Computed flag, this was causing
the plan to not be empty for resources which support tags but none were set, as
no value would be set by flattenAndSetTags.

Setting an empty map instead fixes the issue, ran original failing test and an
update tags test to ensure nothing else was broken.

```
TF_ACC=1 go test ./builtin/providers/azurerm -v -run TestAccAzureRMCdnProfile_basic -timeout 120m
=== RUN   TestAccAzureRMCdnProfile_basic
--- PASS: TestAccAzureRMCdnProfile_basic (208.11s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/azurerm	208.188s
```

builtin/providers/azurerm/tags.go

@@ -64,6 +64,7 @@ func expandTags(tagsMap map[string]interface{}) *map[string]*string {
 
 func flattenAndSetTags(d *schema.ResourceData, tagsMap *map[string]*string) {
 	if tagsMap == nil {
+		d.Set("tags", make(map[string]interface{}))
 		return
 	}