Fix generate(string) docstring and initialize cur_token

- Update docstring: empty prompt is allowed after prefill()
- Initialize cur_token = 0 to silence compiler warnings

This PR was authored with the assistance of Claude.

Author: kirklandsign

extension/llm/runner/multimodal_runner.cpp

@@ -261,7 +261,7 @@ Error MultimodalRunner::generate(
   // Reset internal state and start inference
   stats_->inference_start_ms = time_in_ms();
 
-  uint64_t cur_token;
+  uint64_t cur_token = 0;
   if (!inputs.empty()) {
     // Echo the last text input if enabled
     if (config.echo && inputs.back().is_text()) {

extension/llm/runner/multimodal_runner.h

@@ -111,8 +111,9 @@ class ET_EXPERIMENTAL MultimodalRunner : public IRunner {
 
   /**
    * Generate tokens from a text prompt. Wraps the prompt as a MultimodalInput
-   * and delegates to generate(vector).
-   * @param prompt The text prompt to generate from. Must be non-empty.
+   * and delegates to generate(vector). Empty prompt is allowed if prefill()
+   * was called beforehand.
+   * @param prompt The text prompt to generate from.
    * @param config Generation configuration parameters.
    * @param token_callback Callback function called for each generated token.
    * @param stats_callback Callback function for generation statistics.