`isNullable` check returns `true` for flexible types,
which leads to false positive unsafe invoke error, when
receiver for invoke's property is flexible.
^KT-30695 Fixed
There are several issues with unsafe desugaring for convention calls.
Proper fix is not implemented here (see design proposal KT-30872).
This commit only applies the old logic in the new inference.
^KT-30695 Fixed
Pavel Kirpichenkov