run: Ensure model name/tag only if it's not pulled

[doringeman]

Running a model which has to be pulled will result in a panic.
Without this patch:

$ docker model run ghcr.io/ilopezluna/smollm2 'hi'
Unable to find model 'ghcr.io/ilopezluna/smollm2' locally. Pulling from the server.
Downloaded: 83.75 MB
Model pulled successfully
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/docker/model-cli/commands.newRunCmd.func1(0x140000ffb08, {0x140000d67c0?, 0x0?, 0xb7117dd000000000?})
	/Users/doringeman/workspace/model-cli/commands/run.go:46 +0x61c

[kiview]

note: Eventually we should transition into a development pace, that submits such fixes together with their tests 😉

[glours]

LGTM

[glours]

I wonder if pullModel function should not return a Model struct, so you can update modelDetail 🤔
Anyway it can be done in a follow up PR