[SYCL] Fix macOS build error

[jzc]

Post commit started to failed on 25c3666 (#10620) for macOS build because:

error: no viable overloaded '='
        ReqdWorkGroupSize = NewReqdWorkGroupSize;

note: candidate function not viable: no known conversion from 'llvm::SmallVector<size_t, 3>' (aka 'SmallVector<unsigned long, 3>') to 'const std::optional<llvm::SmallVector<unsigned long long, 3>>' for 1st argument
    _LIBCPP_INLINE_VISIBILITY optional& operator=(const optional&) = default;