[tmva][sofie] Fix compiler warning in tests

The last commit that changed the tests left a compiler warning:
```txt
In file included from /home/rembserj/code/root/root_src/tmva/sofie/test/TestCustomModelsFromONNX.cxx:3:
/home/rembserj/code/root/root_src/tmva/sofie/test/test_helpers.h: In instantiation of ‘OutputType_t runModel(std::string, const std::string&, std::string, const Ts& ...) [with OutputType_t = std::vector<float>; Ts = {}; std::string = std::__cxx11::basic_string<char>]’:
/home/rembserj/code/root/root_src/tmva/sofie/test/TestCustomModelsFromONNX.cxx:318:4:   required from here
  130 |    auto output = runModel<OutputType>(#OutputType, _modelName, "");
      |                  ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/rembserj/code/root/root_src/tmva/sofie/test/test_helpers.h:74:9: warning: variable ‘type_name’ set but not used [-Wunused-but-set-variable]
   74 |    auto type_name = []<typename T>() {
      |         ^~~~~~~~~
/home/rembserj/code/root/root_src/tmva/sofie/test/test_helpers.h:107:9: warning: variable ‘first’ set but not used [-Wunused-but-set-variable]
  107 |    bool first = true;
      |         ^~~~~
cc1plus: note: unrecognized command-line option ‘-Wno-unused-command-line-argument’ may have been intended to silence earlier diagnostics
```
This can be avoided by guarding the parameter pack expension in an if
clause that checks if the size of the parameter pack is not empty.

Author: guitargeek

tmva/sofie/test/test_helpers.h

@@ -70,22 +70,6 @@ runModel(std::string outputTypeName, std::string const &modelName, std::string s
       outputTypeName = "std::tuple<std::vector<float>, std::vector<int64_t>>";
    }
 
-   // Helper: map C++ type -> string used in interpreter
-   auto type_name = []<typename T>() {
-      if constexpr (std::is_same_v<T, int>)
-         return "int";
-      else if constexpr (std::is_same_v<T, std::vector<float>>)
-         return "std::vector<float>";
-      else if constexpr (std::is_same_v<T, std::vector<int>>)
-         return "std::vector<float>";
-      else if constexpr (std::is_same_v<T, std::vector<int64_t>>)
-         return "std::vector<int64_t>";
-      else if constexpr (std::is_same_v<T, std::vector<uint8_t>>)
-         return "std::vector<uint8_t>";
-      else
-         static_assert(!sizeof(T), "Input type not supported");
-   };
-
    std::stringstream cmd;
 
    if (sessionArgs.empty()) {
@@ -104,15 +88,34 @@ runModel(std::string outputTypeName, std::string const &modelName, std::string s
    }
 
    // Emit all inputs to s.infer(...)
-   bool first = true;
-   (
-      [&] {
-         if (!first)
-            cmd << ", ";
-         first = false;
-         cmd << toInterpreter(inputs, type_name.template operator()<Ts>(), true);
-      }(),
-      ...);
+   if constexpr (sizeof...(Ts) > 0) {
+
+      // Helper: map C++ type -> string used in interpreter
+      auto type_name = []<typename T>() {
+         if constexpr (std::is_same_v<T, int>)
+            return "int";
+         else if constexpr (std::is_same_v<T, std::vector<float>>)
+            return "std::vector<float>";
+         else if constexpr (std::is_same_v<T, std::vector<int>>)
+            return "std::vector<float>";
+         else if constexpr (std::is_same_v<T, std::vector<int64_t>>)
+            return "std::vector<int64_t>";
+         else if constexpr (std::is_same_v<T, std::vector<uint8_t>>)
+            return "std::vector<uint8_t>";
+         else
+            static_assert(!sizeof(T), "Input type not supported");
+      };
+
+      bool first = true;
+      (
+         [&] {
+            if (!first)
+               cmd << ", ";
+            first = false;
+            cmd << toInterpreter(inputs, type_name.template operator()<Ts>(), true);
+         }(),
+         ...);
+   }
 
    cmd << R"();
    std::swap(output, *)"