Fix unused object parameters in empty structure serialization

Extends the unused variable fix to handle function parameters in addition to union match variables. For empty structures, both object and input parameters are prefixed with underscore to prevent unused variable warnings in CI.

Addresses CI failure in shape_player_action.rs

Author: vcjana

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/JsonSerializerGenerator.kt

@@ -355,19 +355,22 @@ class JsonSerializerGenerator(
                             Attribute.AllowUnusedVariables.render(this)
                         }
                     }
+                val objectParam = if (members.isEmpty()) "_object" else "object"
+                val inputParam = if (members.isEmpty()) "_input" else "input"
                 rustBlockTemplate(
                     """
                     pub fn $fnName(
-                        #{AllowUnusedVariables:W} object: &mut #{JsonObjectWriter},
-                        #{AllowUnusedVariables:W} input: &#{StructureSymbol},
+                        #{AllowUnusedVariables:W} $objectParam: &mut #{JsonObjectWriter},
+                        #{AllowUnusedVariables:W} $inputParam: &#{StructureSymbol},
                     ) -> #{Result}<(), #{Error}>
                     """,
                     "StructureSymbol" to symbolProvider.toSymbol(context.shape),
                     "AllowUnusedVariables" to allowUnusedVariables,
                     *codegenScope,
                 ) {
                     for (member in members) {
-                        serializeMember(MemberContext.structMember(inner, member, symbolProvider, jsonName))
+                        val actualInner = inner.copy(objectName = objectParam, localName = inputParam)
+                        serializeMember(MemberContext.structMember(actualInner, member, symbolProvider, jsonName))
                     }
                     rust("Ok(())")
                 }