awaitSingleOrNull returns not nullable value

[Lewik]

kotlinx.coroutines/reactive/kotlinx-coroutines-reactive/src/Await.kt

Line 107 in edd584a

public suspend fun <T> Publisher<T>.awaitSingleOrNull(): T = awaitOne(Mode.SINGLE_OR_DEFAULT)

awaitSingleOrNull returns not nullable value and

@throws NoSuchElementException if publisher does not emit any value

Also, behaviour differs from the std function:

@throws IllegalArgumentException if publisher emits more than one value

/**
 * Returns single element, or `null` if the list is empty or has more than one element.
 */
public fun <T> List<T>.singleOrNull()

[qwwdfsad]

Note that we also fixed the corresponding Flow operator for the same reason: #2289