New J2K: separate nullability inference from common one & nullability bug fixes

It will be needed for structure mutability inference

#KT-21467 fixed
#KT-32609 fixed
#KT-32572 fixed
#KT-24677 fixed

nj2k/testData/inference/nullability/nullAsInitializer.kt

@@ -0,0 +1,5 @@
+val x: /*T0@*/Int? = null/*NULL!!U*/
+fun foo(p: /*T1@*/Int? = null/*NULL!!U*/) {}
+
+//UPPER <: T0 due to 'INITIALIZER'
+//UPPER <: T1 due to 'INITIALIZER'