IC: Fix regression in detecting constants in KotlinClassInfo.kt

A constant is a static final field with non-null value. In a previous
commit (0b09be7), we accidentally removed the *non-null value*
filter when looking for constants in the bytecode.

This commit re-adds that filter to make sure the detection is correct.

Test: Added KotlinOnlyClasspathChangesComputerTest.testDelegatedProperties

^KT-58986: Fixed

compiler/incremental-compilation-impl/testData/org/jetbrains/kotlin/incremental/classpathDiff/ClasspathChangesComputerTest/KotlinOnly/testDelegatedProperties/src/current-classpath/com/example/FileFacade.kt

@@ -0,0 +1,15 @@
+package com.example
+
+import kotlin.reflect.KProperty
+
+var delegatedProperty: String by Delegate() // Added
+
+class Delegate {
+
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
+        return "<Delegated>"
+    }
+
+    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
+    }
+}

compiler/incremental-compilation-impl/testData/org/jetbrains/kotlin/incremental/classpathDiff/ClasspathChangesComputerTest/KotlinOnly/testDelegatedProperties/src/previous-classpath/com/example/FileFacade.kt

@@ -0,0 +1,13 @@
+package com.example
+
+import kotlin.reflect.KProperty
+
+class Delegate {
+
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
+        return "<Delegated>"
+    }
+
+    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
+    }
+}