[jsscripting] Add an alternative MIME type/alias (#13851)

[jsscripting] Add an alternative MIME type/alias (#13851)

The alternative MIME type text/javascript is also used by Nashorn, therefore we add the same extension as for the main MIME type.
The alias is introduced to simplify the usage in SCRIPT transformation profiles.

* Fix syntax problems in the bndtools file
* Add null check to avoid potential NPE
  In some cases during testing, a NPE occured, this should solve it.

Signed-off-by: Florian Hotze <florianh_dev@icloud.com>