sim: Clarify condition with is_empty()
sim: Clarify condition with is_empty()

Clippy suggests `.is_empty()` over `.len() == 0`.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot