Clean up JVMTI DDMS extension function.

We change it to not return failure (and print a warning) if the chunk
handler returns an empty chunk. This is a surprisingly common result
in real-world code and was causing significant log-spam.

Bug: 70559172
Test: ./test.py --host -j50
Change-Id: I1ba0f43cb2e834b09f51db75ec9100d97e916b62