Tailscale SaaS accepts grants with empty src=[] or dst=[] arrays,
producing no filter rules for any node. Headscale previously rejected
these with validation errors.
Remove the empty source/destination validation checks and add an
early return in compileGrantWithAutogroupSelf when the grant has
literally empty Sources or Destinations arrays. This is distinct
from sources that resolve to empty (e.g., group:empty) where
Tailscale still produces CapGrant rules with empty SrcIPs.
Updates #2180
84210c03bb