return nan when quantileExact with empty float column

[sundy-li]

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I think quantileExact function should return nan when quantileExact with empty float column like quantile function.

[sundy-li]

Such as

SELECT quantileExactIf(number, number > 100)
FROM numbers(90)

┌─quantileExactIf(number, greater(number, 100))─┐
│                                             0 │
└───────────────────────────────────────────────┘

SELECT quantileExactIf(toFloat64(number), number > 100)
FROM numbers(90)

┌─quantileExactIf(toFloat64(number), greater(number, 100))─┐
│                                                      nan │
└──────────────────────────────────────────────────────────┘

SELECT quantileIf(number, number > 100)
FROM numbers(90)

┌─quantileIf(number, greater(number, 100))─┐
│                                      nan │
└──────────────────────────────────────────┘

[alexey-milovidov]

A test?

[alexey-milovidov]

Thank you!

[alesapin]

It seems like this PR failed test 00606_quantiles_and_nans https://travis-ci.org/yandex/ClickHouse/jobs/415344194.