fix(cli): skip empty YAML documents during apply

[ktsoator]

ax apply fails with "missing kind" when a manifest contains an empty YAML document, such as a trailing "---". Skip empty documents so subsequent resources are processed, while preserving errors for explicit null values and invalid manifests.

To reproduce, save the following as workspaces.yaml:

apiVersion: ax.io/v1alpha1
kind: Workspace
metadata:
  name: first
spec: {}
---
# empty document
---
apiVersion: ax.io/v1alpha1
kind: Workspace
metadata:
  name: second
spec: {}

With an AX server available, run:

ax apply -f workspaces.yaml

Verified using the actual CLI against a local AX gRPC server with an in-memory store.

Before (process exit code: 1):

workspace.ax.io/first created
Error: applying document 2: missing kind

After (process exit code: 0):

workspace.ax.io/first created
workspace.ax.io/second created

Added regression tests. Verified with make test, make build, and go mod tidy with no module changes.