Weaver returns null instead of original bytes for unwoven classes

Weaver returns null instead of original bytes for unwoven classes

This change makes sense independently of #277, but also enables using

  - cp "my.jar;aspectjweaver.jar"
  -XX:+AllowArchivingWithJavaAgent
  -javaagent:aspectjweaver.jar

while creating a CDS archive. Afterward, the application can be run in
its woven state from the CDS archive even without '-javaagent', because
the byte code was archived in its woven state ("poor man's AJC"). See
https://github.com/eclipse-aspectj/aspectj/issues/277#issuecomment-1931142753
for details.

Fixes #277.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>