python: Make DOT config generation optional

python: Make DOT config generation optional

By default, DOT configs are always generated when pydot is present.
This change allows a user to pass an empty --dot-config='' to disable
generating the DOT configuration. This can be useful to save space, or
to reduce Gem5 startup time when running many small regression tests.

This brings the behavior in-line with providing an empty
--dump_config='' and/or --json_config='' which similarly disables
generation of those output files.

Change-Id: I5bf39fda0409b948a8d14f3afa95db8fc78de6ee
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29232
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>