[clang-tidy] Fix failing test after #80864 (#81171)

The following test case in
`clang-tools-extra/test/clang-tidy/infrastructure/diagnostic.cpp` is
failing:
```
#ifdef PR64602 // Should not crash
template <class T = void>
struct S
{
    auto foo(auto);
};

template <>
auto S<>::foo(auto)
{
    return 1;
}
// CHECK8: error: template parameter list matching the non-templated nested type 'S<>' should be empty ('template<>') [clang-diagnostic-error]
#endif
```

#80864 fixes a bug where we would (incorrectly) append invented template
parameters to empty template parameter lists, which causes this test to
fail.