inspect: do not display platforms field if empty

[crazy-max]

Before:

$ docker buildx inspect builder
Name:          builder
Driver:        docker-container
Last Activity: 2023-09-12 20:09:21 +0000 UTC

Nodes:
Name:           builder0
Endpoint:       unix:///var/run/docker.sock
Driver Options: env.BUILDKIT_STEP_LOG_MAX_SIZE="10485760" env.BUILDKIT_STEP_LOG_MAX_SPEED="10485760" env.JAEGER_TRACE="localhost:6831" image="moby/buildkit:master" network="host"
Status:         stopped
Flags:          --debug --allow-insecure-entitlement security.insecure --allow-insecure-entitlement network.host
Platforms:

Now:

$ docker buildx inspect builder
Name:          builder
Driver:        docker-container
Last Activity: 2023-09-12 20:09:21 +0000 UTC

Nodes:
Name:           builder0
Endpoint:       unix:///var/run/docker.sock
Driver Options: image="moby/buildkit:master" network="host" env.BUILDKIT_STEP_LOG_MAX_SIZE="10485760" env.BUILDKIT_STEP_LOG_MAX_SPEED="10485760" env.JAEGER_TRACE="localhost:6831"
Status:         stopped
Flags:          --debug --allow-insecure-entitlement security.insecure --allow-insecure-entitlement network.host