[HST Spring Support] Make HstRepositoryResourceBundleMessageSource#getStringOrNull(ResourceBundle bundle, String key) protected

For a specific usecase we need to override HstRepositoryResourceBundleMessageSource #private String getStringOrNull(ResourceBundle bundle, String key), this method should be made protected or removed completely, as the ResourceBundleMessageSource from Spring actually has the same method where it is protected already. Only difference is a #containsKey check. 

It would be much appreciated if this can be fixed asap. This should be possible in my opinion, as the HstRepositoryResourceBundleMessageSource is in a seperate forge plugin.

Also in IntelliJ IDEA an error is thrown at the moment, see the attached image for more details.