For example, if I use UInt8 constant for if condition then only one branch is evaluated:
```sql
SELECT if(1, 'success', throwIf(1, 'Executing FALSE branch'))

┌─'success'─┐
│ success   │
└───────────┘

1 rows in set. Elapsed: 0.003 sec.
```

But if I use some constant expression, then both branches are evaluated:
```sql
SELECT if(empty(''), 'success', throwIf(1, 'Executing FALSE branch'))

0 rows in set. Elapsed: 0.003 sec.

Received exception from server (version 21.12.2):
Code: 395. DB::Exception: Received from localhost:9000. DB::Exception: Executing FALSE branch: While processing if(empty(''), 'success', throwIf(1, 'Executing FALSE branch')). (FUNCTION_THROW_IF_VALUE_IS_NON_ZERO)
```
{
    size_t size = cond.size();
    bool a_is_short = a.size() < size;
    if (a_is_short)
    {
        size_t a_index = 0;
        for (size_t i = 0; i < size; ++i)
            res[i] = cond[i] ? static_cast<ResultType>(a[a_index++]) : static_cast<ResultType>(b);
    }
    else
    {
        for (size_t i = 0; i < size; ++i)
            res[i] = cond[i] ? static_cast<ResultType>(a[i]) : static_cast<ResultType>(b);
    }
}
template <typename ArrayCond, typename A, typename ArrayB, typename ArrayResult, typename ResultType>
static inline void fillConstantVector(const ArrayCond & cond, A a, const ArrayB & b, ArrayResult & res)