Improve the output message of notation inspect images without signatures

[yizha1]

What is the areas you experience the issue in?

Notation CLI

What is not working as expected?

Use notation inspect images without any signatures, the output message is not user friendly.

notation inspect localhost:5000/nginx:latest
Warning: Always inspect the artifact using digest(@sha256:...) rather than a tag(:latest) because resolved digest may not point to the same signed artifact, as tags are mutable.
Inspecting all signatures for signed artifact
localhost:5000/nginx@sha256:bfb112db4075460ec042ce13e0b9c3ebd982f93ae0be155496d050bb70006750
└── application/vnd.cncf.notary.signature

What did you expect to happen?

The output message should tell the users that there are no signatures associated with the image.

How can we reproduce it?

Run notation inspect against images without any signatures.

Describe your environment

WSL

What is the version of your Notation CLI or Notation Library?

v1.0.0-rc.3

[yizha1]

Improve the output when inspecting images without signatures.