[Commonizer] Make CirProperty.compileTimeInitializer non-nullable

There is effectively no difference between null and CirConstantValue.NullValue recorded in compileTimeInitializer.

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/cir/CirProperty.kt

@@ -15,5 +15,5 @@ interface CirProperty : CirFunctionOrProperty, CirLiftedUpDeclaration {
     val setter: CirPropertySetter?
     val backingFieldAnnotations: List<CirAnnotation>
     val delegateFieldAnnotations: List<CirAnnotation>
-    val compileTimeInitializer: CirConstantValue<*>?
+    val compileTimeInitializer: CirConstantValue<*>
 }

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/cir/factory/CirPropertyFactory.kt

@@ -6,9 +6,9 @@
 package org.jetbrains.kotlin.descriptors.commonizer.cir.factory
 
 import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
+import org.jetbrains.kotlin.descriptors.DescriptorVisibility
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.PropertyDescriptor
-import org.jetbrains.kotlin.descriptors.DescriptorVisibility
 import org.jetbrains.kotlin.descriptors.commonizer.cir.*
 import org.jetbrains.kotlin.descriptors.commonizer.cir.impl.CirPropertyImpl
 import org.jetbrains.kotlin.descriptors.commonizer.utils.compactMap
@@ -20,7 +20,7 @@ object CirPropertyFactory {
                 constantValue = constantValue,
                 owner = source,
             )
-        }
+        } ?: CirConstantValue.NullValue
 
         return create(
             annotations = source.annotations.compactMap(CirAnnotationFactory::create),
@@ -65,7 +65,7 @@ object CirPropertyFactory {
         setter: CirPropertySetter?,
         backingFieldAnnotations: List<CirAnnotation>,
         delegateFieldAnnotations: List<CirAnnotation>,
-        compileTimeInitializer: CirConstantValue<*>?
+        compileTimeInitializer: CirConstantValue<*>
     ): CirProperty {
         return CirPropertyImpl(
             annotations = annotations,

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/cir/impl/CirPropertyImpl.kt

@@ -29,7 +29,7 @@ data class CirPropertyImpl(
     override val setter: CirPropertySetter?,
     override val backingFieldAnnotations: List<CirAnnotation>,
     override val delegateFieldAnnotations: List<CirAnnotation>,
-    override val compileTimeInitializer: CirConstantValue<*>?
+    override val compileTimeInitializer: CirConstantValue<*>
 ) : CirProperty {
     // const property in "common" fragment is already lifted up
     override val isLiftedUp get() = isConst

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/core/PropertyCommonizer.kt

@@ -47,7 +47,7 @@ class PropertyCommonizer(classifiers: CirKnownClassifiers) : AbstractFunctionOrP
             setter = setter,
             backingFieldAnnotations = emptyList(),
             delegateFieldAnnotations = emptyList(),
-            compileTimeInitializer = constCompileTimeInitializer
+            compileTimeInitializer = constCompileTimeInitializer ?: CirConstantValue.NullValue
         )
     }
 
@@ -55,7 +55,7 @@ class PropertyCommonizer(classifiers: CirKnownClassifiers) : AbstractFunctionOrP
         super.initialize(first)
 
         constCommonizationState = if (first.isConst) {
-            first.compileTimeInitializer?.let(::ConstSameValue) ?: NonConst
+            first.compileTimeInitializer.takeIf { it != CirConstantValue.NullValue }?.let(::ConstSameValue) ?: NonConst
         } else {
             NonConst
         }
@@ -105,11 +105,17 @@ class PropertyCommonizer(classifiers: CirKnownClassifiers) : AbstractFunctionOrP
 
     private sealed class ConstCommonizationState {
         object NonConst : ConstCommonizationState()
+
         abstract class Const : ConstCommonizationState() {
             val properties: MutableList<CirProperty> = mutableListOf()
         }
 
-        class ConstSameValue(val compileTimeInitializer: CirConstantValue<*>) : Const()
+        class ConstSameValue(val compileTimeInitializer: CirConstantValue<*>) : Const() {
+            init {
+                check(compileTimeInitializer != CirConstantValue.NullValue)
+            }
+        }
+
         class ConstMultipleValues(previous: ConstSameValue) : Const() {
             init {
                 properties += previous.properties