Fix compile error in binaryop/compiled/util.cpp (NVIDIA#10209)

This PR fixes a compile error in `binaryop/compiled/util.cpp`, when a function with return type `std::optional` return `{}` instead of `std::nullopt`. As such, nvcc 11.5/g++-10.3 yell:
```
../src/binaryop/compiled/util.cpp: In function 'std::optional<cudf::data_type> cudf::binops::compiled::get_common_type(cudf::data_type, cudf::data_type, cudf::data_type)':
../src/binaryop/compiled/util.cpp:48:15: error: '<anonymous>' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   48 |       return {};
      |               ^
```

Authors:
  - Nghia Truong (https://github.com/ttnghia)

Approvers:
  - Robert Maynard (https://github.com/robertmaynard)
  - Karthikeyan (https://github.com/karthikeyann)
  - MithunR (https://github.com/mythrocks)
  - Bradley Dice (https://github.com/bdice)

URL: rapidsai/cudf#10209

cpp/src/binaryop/compiled/util.cpp

@@ -45,7 +45,11 @@ struct common_type_functor {
         // Eg. d=t-t
         return data_type{type_to_id<TypeCommon>()};
       }
-      return {};
+
+      // A compiler bug may cause a compilation error when using empty initializer list to construct
+      // an std::optional object containing no `data_type` value. Therefore, we should explicitly
+      // return `std::nullopt` instead.
+      return std::nullopt;
     }
   };
   template <typename TypeLhs, typename TypeRhs>