**Describe what's wrong**

It's not possible to initialize aggregation function from nullable types.

**Does it reproduce on recent release?**

Yes, 
ClickHouse version 21.10

```
SELECT initializeAggregation('uniqExactState', toNullable(''))


0 rows in set. Elapsed: 0.003 sec.

Received exception from server (version 21.10.1):
Code: 43. DB::Exception: Received from localhost:9000. DB::Exception: Nested type AggregateFunction(uniqExact, String) cannot be inside Nullable type: While processing initializeAggregation('uniqExactState', toNullable('')). (ILLEGAL_TYPE_OF_ARGUMENT)


SELECT initializeAggregation('uniqExactState', toNullable(1))


Received exception from server (version 21.10.1):
Code: 43. DB::Exception: Received from localhost:9000. DB::Exception: Nested type AggregateFunction(uniqExact, UInt8) cannot be inside Nullable type: While processing initializeAggregation('uniqExactState', toNullable(1)). (ILLEGAL_TYPE_OF_ARGUMENT)
```


**Expected behavior**

It should be possible to initialize aggregation function from nullable types.


**Additional context**

WA

```
SELECT arrayReduce('uniqExactState', [toNullable('')])

Query id: 29fc6760-78be-4f28-be5f-f1bbe12f04fc

┌─arrayReduce('uniqExactState', array(toNullable('')))─┐
│ 2���V���嵶����                                                │
└──────────────────────────────────────────────────────┘
```

#include <Functions/IFunction.h>
#include <Functions/FunctionFactory.h>
#include <Functions/FunctionHelpers.h>
#include <Columns/ColumnString.h>
#include <Columns/ColumnAggregateFunction.h>
#include <AggregateFunctions/AggregateFunctionFactory.h>
#include <AggregateFunctions/AggregateFunctionState.h>
#include <AggregateFunctions/IAggregateFunction.h>
#include <AggregateFunctions/parseAggregateFunctionParameters.h>
#include <Common/Arena.h>
#include <base/scope_guard_safe.h>
namespace DB
{
namespace ErrorCodes
{
    extern const int NUMBER_OF_ARGUMENTS_DOESNT_MATCH;
    extern const int ILLEGAL_COLUMN;
    extern const int ILLEGAL_TYPE_OF_ARGUMENT;
    extern const int BAD_ARGUMENTS;
}
namespace
{
class FunctionInitializeAggregation : public IFunction, private WithContext