genai: check for nil value from service (#57)

Avoid dereferencing a nil.
This may fix an actual bug, but even if it doesn't, it's a good idea to
assume as little as possible about values returned from the service.

For #55.

genai/chat.go

@@ -60,6 +60,9 @@ func (cs *ChatSession) SendMessageStream(ctx context.Context, parts ...Part) *Ge
 func (cs *ChatSession) addToHistory(cands []*Candidate) bool {
  if len(cands) > 0 {
  c := cands[0].Content
+ if c == nil {
+ return false
+ }
  c.Role = roleModel
  cs.History = append(cs.History, c)
  return true