Pass :system message when neither :prompt nor :session is there

[vderyagin]

before:

#+begin_src gptel :system "RESPOND IN ALL CAPS" :dry-run yes
tell me a joke
#+end_src

#+RESULTS:
: (:model "llama3.1:latest" :messages [(:role "user" :content "tell me a joke")]
:         :stream :json-false :options (:temperature 1.0))

after:

#+begin_src gptel :system "RESPOND IN ALL CAPS" :dry-run yes
tell me a joke
#+end_src

#+RESULTS:
: (:model "llama3.1:latest" :messages
:         [(:role "system" :content "RESPOND IN ALL CAPS")
:          (:role "user" :content "tell me a joke")]
:         :stream :json-false :options (:temperature 1.0))