googleai: panic on system-only message list in GenerateContent

generateFromMessages panics with index out of range [-1] when all input messages are system messages (history becomes empty before selecting the request message).

Repro shape:

• messages contain only role=system
• call GenerateContent (including stream mode)
• panic at history[n-1]

Stack seen from downstream integration: github.com/tmc/langchaingo/llms/googleai.generateFromMessages(...) llms/googleai/googleai.go:333

Suggested fix: return an error for empty non-system history, or gracefully route to single-message path.