Treat `runtime_kwargs` as provided whenever it is not `None`.
Previously an empty dict bypassed placement validation and was
silently ignored when `an` or `portal` selected an existing runtime.
Reject both placement modes before actor startup for empty and
configured runtime kwargs while preserving empty-dict use when
`to_actor.run()` owns its private runtime.
Caught-during: review remediation
Found-via: `/code-review` P3 option-validation finding
Review: PR #481 (opencode)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-4956692120
Prompt-IO: ai/prompt-io/opencode/20260819T020757Z_b38efed7_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi