FIR: fix nullability loss on substitution

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeInferenceContext.kt

@@ -271,7 +271,7 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext,
             TypeSubstitutorMarker {
             override fun substituteType(type: ConeKotlinType): ConeKotlinType? {
                 val new = map[type.typeConstructor()] ?: return null
-                return new as ConeKotlinType
+                return makeNullableIfNeed(type.isMarkedNullable, new as ConeKotlinType)
             }
         }
     }

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/substitution/ConeSubstitutor.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.resolve.substitution
 
+import org.jetbrains.kotlin.fir.resolve.withNullability
 import org.jetbrains.kotlin.fir.symbols.ConeTypeParameterSymbol
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.fir.types.impl.ConeAbbreviatedTypeImpl
@@ -44,6 +45,11 @@ abstract class AbstractConeSubstitutor : ConeSubstitutor {
 
     abstract fun substituteType(type: ConeKotlinType): ConeKotlinType?
 
+    fun makeNullableIfNeed(isNullable: Boolean, type: ConeKotlinType?): ConeKotlinType? {
+        if (!isNullable) return type
+        return type?.withNullability(ConeNullability.NULLABLE)
+    }
+
     override fun substituteOrSelf(type: ConeKotlinType): ConeKotlinType {
         return substituteOrNull(type) ?: type
     }
@@ -124,6 +130,6 @@ class ConeSubstitutorByMap(val substitution: Map<ConeTypeParameterSymbol, ConeKo
 
     override fun substituteType(type: ConeKotlinType): ConeKotlinType? {
         if (type !is ConeTypeParameterType) return null
-        return substitution[type.lookupTag]
+        return makeNullableIfNeed(type.isMarkedNullable, substitution[type.lookupTag])
     }
 }