only print manual newline with reverse prompt

fix formatting of reverse prompts so they don't end up at the end of the current line while not introducing unnecessary new lines otherwise

main.cpp

@@ -1089,7 +1089,9 @@ int main(int argc, char ** argv) {
                 is_interacting = true;
                 embd.back() = NEWLINE_TOKEN_ID;
                 last_n_tokens.back() = NEWLINE_TOKEN_ID;
+                if (params.antiprompt.size() != 0) {
                    fprintf(stderr, "\n");
+            }
             } else {
                 fprintf(stderr, " [end of text]\n");
                 break;