fix(llm): handle empty custom LLMs gracefully

Check if custom LLMs string is empty before attempting to decode, returning an empty list if true. This prevents unnecessary exceptions when no custom LLMs are configured.

Author: phodal

core/src/main/kotlin/cc/unitmesh/devti/llm2/model/LlmConfig.kt

@@ -1,6 +1,7 @@
 package cc.unitmesh.devti.llm2.model
 
 import cc.unitmesh.devti.settings.AutoDevSettingsState
+import com.intellij.openapi.diagnostic.logger
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
 import kotlin.text.ifEmpty
@@ -23,8 +24,12 @@ data class LlmConfig(
 ) {
     companion object {
         fun load(): List<LlmConfig> {
+            val llms = AutoDevSettingsState.getInstance().customLlms.trim()
+            if (llms.isEmpty()) {
+                return emptyList()
+            }
+
             val configs: List<LlmConfig> = try {
-                val llms = AutoDevSettingsState.getInstance().customLlms
                 Json.decodeFromString(llms)
             } catch (e: Exception) {
                 throw Exception("Failed to load custom llms: $e")