mem-cache: Do not try to compress dataless packets

mem-cache: Do not try to compress dataless packets

Fix filling blocks so that packets that do not contain data do not
generate a compression attempt. This can happen, for example, with
invalidation responses, which will trigger a packet data access
assertion.

Change-Id: I2a1e7983657f6e5e770b148ab62c9de9ac3986ac
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22164
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>