fix: avoid crash when the lora file is not found using immediately mode (leejet#1022)

Author: leejet

stable-diffusion.cpp

@@ -991,6 +991,9 @@ class StableDiffusionGGML {
             int64_t t0 = ggml_time_ms();
 
             auto lora = load_lora_model_from_file(kv.first, kv.second, backend);
+            if (!lora || lora->lora_tensors.empty()) {
+                continue;
+            }
             lora->apply(tensors, version, n_threads);
             lora->free_params_buffer();