fix(docker): avoid empty string in bake's arguments when targets not specified

[kazuki0824]

Description

Pre-review checklist for the PR author

The PR author must check the checkboxes below when creating the PR.

• I've confirmed the contribution guidelines.
• The PR follows the pull request guidelines.

In-review checklist for the PR reviewers

The PR reviewers must check the checkboxes below before approval.

• The PR follows the pull request guidelines.

Post-review checklist for the PR author

The PR author must check the checkboxes below before merging.

• There are no open discussions or they are tracked via tickets.

After all checkboxes are checked, anyone who has write access can merge the PR.

[HansRobo]

@kazuki0824 Thank you for your contribution!
Could you describe what happens without this PR, and how did you fix it?

[HansRobo]

@kenji-miyake Can you review this?

[kazuki0824]

I found another solution using array expansion: by making targets an array, the contents can properly be considered as multiple values through "${targets[@]}" instead of a single string. See f44fdee

[kazuki0824]

@kazuki0824 Thank you for your contribution! Could you describe what happens without this PR, and how did you fix it?

@HansRobo

Without this PR, docker/build.sh will fail.
If the option --no-prebuilt is not specified, then $targets will be set as follows:

autoware/docker/build.sh

Lines 38 to 44 in 3555383

	# Set prebuilt options
	if [ "$option_no_prebuilt" = "true" ]; then
	targets="devel"
	else
	# default targets include devel and prebuilt
	targets=""
	fi

and then an empty string will be added to the bottom of the argv of the bake :

autoware/docker/build.sh

Lines 66 to 76 in 3555383

	docker buildx bake --no-cache --load --progress=plain -f "$SCRIPT_DIR/autoware-universe/docker-bake.hcl" \
	--set "*.context=$WORKSPACE_ROOT" \
	--set "*.ssh=default" \
	--set "*.platform=$platform" \
	--set "*.args.ROS_DISTRO=$rosdistro" \
	--set "*.args.BASE_IMAGE=$base_image" \
	--set "*.args.PREBUILT_BASE_IMAGE=$prebuilt_base_image" \
	--set "*.args.SETUP_ARGS=$setup_args" \
	--set "devel.tags=ghcr.io/autowarefoundation/autoware-universe:$rosdistro-latest$image_name_suffix" \
	--set "prebuilt.tags=ghcr.io/autowarefoundation/autoware-universe:$rosdistro-latest-prebuilt$image_name_suffix" \
	"$targets"