Adding assertions to records results in invalid class files

[xzel23]

Injecting the static initializer into the class file seems to work, but the generated classfile cannot be loaded into the JVM, see javassist#481.

[xzel23]

added workaround: check for null and throw NPE instead of generating assertions when class is a record.

[xzel23]

This might be related to https://bugs.openjdk.org/browse/JDK-8247517. Cabe currently injects the field and a static initializer block. Maybe there's another way to do it in Javassist.