Fix resource Read inconsistencies

[yannickstruyf3]

The Read function of some (for example nutanix_category_key and nutanix_category_value ) resources incorrectly use following code block:

	if err != nil {
		if strings.Contains(fmt.Sprint(err), "ENTITY_NOT_FOUND") {
			d.SetId("")
                         /////////
                        // Requires return nil
                        /////////
		}
		return err
	}

Without return nil in case the entity is not found, terraform will throw an error in case the resource is deleted without terraform intervention (for example deletion via UI or manual API call).