length check added before type assertion to avoid panic (#4184) (#2696)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/4184.txt

@@ -0,0 +1,4 @@
+```release-note:bug
+storage - added length check for `cors` for the resource `resource_storage_bucket`
+
+```

google-beta/resource_storage_bucket.go

@@ -844,6 +844,9 @@ func resourceStorageBucketStateImporter(d *schema.ResourceData, meta interface{}
 
 func expandCors(configured []interface{}) []*storage.BucketCors {
 	corsRules := make([]*storage.BucketCors, 0, len(configured))
+	if len(corsRules) == 0 {
+		return nil
+	}
 	for _, raw := range configured {
 		data := raw.(map[string]interface{})
 		corsRule := storage.BucketCors{