refactor(kotlin): update null safety in GoFullIdentListener.kt

Avoid potential null pointer exceptions by using safe call operator on typeDef references within the buildTypeSpec function. This change ensures the code is more robust when dealing with optional types.

chapi-ast-go/src/main/kotlin/chapi/ast/goast/GoFullIdentListener.kt

@@ -176,7 +176,7 @@ class GoFullIdentListener(var fileName: String) : GoAstListener() {
     private fun buildTypeSpec(typeSpec: GoParser.TypeSpecContext) {
         val typeDef = typeSpec.typeDef()
         val identifyName = typeDef?.IDENTIFIER()?.text ?: ""
-        typeDef.type_().typeLit()?.let {
+        typeDef?.type_()?.typeLit()?.let {
             when (val typeChild = it.getChild(0)) {
                 is GoParser.StructTypeContext -> {
                     buildStruct(identifyName, typeChild)