vad : return early if no vad segments are detected

This commit adds a check to `whisper_full_with_state` and if no VAD
segments are detected, the function will return early.

The motivation for this is that if no VAD segments are detected, the
function will not have any samples to process which can happen if an
audio sample does not contain any speech. I did not test this previously
and only discovered this when updating the stream example.

Author: danbev

src/whisper.cpp

@@ -6746,6 +6746,9 @@ int whisper_full_with_state(
             WHISPER_LOG_ERROR("%s: failed to compute VAD\n", __func__);
             return -1;
         }
+        if (vad_n_samples == 0) {
+            return 0;
+        }
         process_samples = vad_samples.data();
         n_process_samples = vad_n_samples;
     }