[NFC][HW] Fix parsing of nullary hw.triggered ops (#7291)

Make the variadic inputs argument of the hw.triggered operation optional in the declarative assembly format. Without this change the generated parser fails on nullary instances due to the empty/missing type list.

include/circt/Dialect/HW/HWStructure.td

@@ -714,7 +714,7 @@ def TriggeredOp : HWOp<"triggered", [
   let results = (outs);
 
   let assemblyFormat = [{
-    $event $trigger  `(` $inputs `)` `:` type($inputs) $body attr-dict
+    $event $trigger  (`(` $inputs^ `)` `:` type($inputs))? $body attr-dict
   }];
 
   let extraClassDeclaration = [{

test/Conversion/HWToSV/test_trigger.mlir

@@ -9,3 +9,13 @@ hw.module @foo(in %trigger : i1, in %in : i32) {
       "some.user" (%arg0) : (i32) -> ()
   }
 }
+
+hw.module @bar(in %trigger : i1) {
+  // CHECK:       sv.always posedge %trigger {
+  // CHECK-NEXT:    "some.user"() : () -> ()
+  // CHECK-NEXT:  }
+  hw.triggered posedge %trigger {
+      "some.user"() : () -> ()
+  }
+
+}