> So, can we assure that there is a bug in JBoss regarding to this issue?

Without changing the method to check for null value - no. If you want to
build driver from source, just add on the beginning of the
FBManagedConnectionFactory.setEncoding(String value) method

if (encoding == null)
throw new NullPointerException("Encoding cannot be null.");

If then JBoss shows this exception, we can talk about the error in JBoss.

Roman