[FIR] Fix nullability of function type created from custom function type

#KT-64706 Fixed

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/FunctionalTypeUtils.kt

@@ -113,7 +113,7 @@ private fun ConeKotlinType.createFunctionTypeWithNewKind(session: FirSession, ki
     val functionTypeId = ClassId(kind.packageFqName, kind.numberedClassName(expandedType.typeArguments.size - 1))
     return functionTypeId.toLookupTag().constructClassType(
         expandedType.typeArguments,
-        isNullable = false,
+        isNullable = expandedType.isNullable,
         attributes = expandedType.attributes
     )
 }

plugins/fir-plugin-prototype/testData/box/nullableComposableParameterAcrossModules.fir.txt

@@ -0,0 +1,18 @@
+Module: a
+FILE: a.kt
+    public final fun RMenuItem(onClick: R|(() -> kotlin/Unit)?| = Null(null), trailing: R|@R|org/jetbrains/kotlin/fir/plugin/MyComposable|()  some/MyComposableFunction0<kotlin/Unit>?| = Null(null)): R|kotlin/Unit| {
+    }
+Module: b
+FILE: b.kt
+    public final fun bar(onClick: R|(() -> kotlin/Unit)?| = Null(null)): R|kotlin/Unit| {
+        R|/RMenuItem|(onClick = R|<local>/onClick|, trailing = R|<local>/onClick|?.{ $subj$.R|kotlin/let|<R|() -> kotlin/Unit|, R|@R|org/jetbrains/kotlin/fir/plugin/MyComposable|()  some/MyComposableFunction0<kotlin/Unit>?|>(<L> = let@fun <anonymous>(it: R|() -> kotlin/Unit|): R|@R|org/jetbrains/kotlin/fir/plugin/MyComposable|()  some/MyComposableFunction0<kotlin/Unit>?| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^ let@fun <anonymous>(): R|kotlin/Unit| <inline=Unknown>  {
+                ^@let Unit
+            }
+
+        }
+        ) })
+    }
+    public final fun box(): R|kotlin/String| {
+        ^box String(OK)
+    }

plugins/fir-plugin-prototype/testData/box/nullableComposableParameterAcrossModules.kt

@@ -0,0 +1,25 @@
+// ISSUE: KT-64994
+
+// MODULE: a
+// FILE: a.kt
+import org.jetbrains.kotlin.fir.plugin.MyComposable
+
+fun RMenuItem(
+    onClick: (() -> Unit)? = null,
+    trailing: (@MyComposable () -> Unit)? = null,
+) {}
+
+// MODULE: b(a)
+// FILE: b.kt
+import org.jetbrains.kotlin.fir.plugin.MyComposable
+
+fun bar(onClick: (() -> Unit)? = null) {
+    RMenuItem(
+        onClick = onClick,
+        trailing = onClick?.let {
+            { }
+        }
+    )
+}
+
+fun box() = "OK"

plugins/fir-plugin-prototype/tests-gen/org/jetbrains/kotlin/fir/plugin/runners/FirLightTreePluginBlackBoxCodegenTestGenerated.java

@@ -97,6 +97,12 @@ public class FirLightTreePluginBlackBoxCodegenTestGenerated extends AbstractFirL
         runTest("plugins/fir-plugin-prototype/testData/box/newSupertype.kt");
     }
 
+    @Test
+    @TestMetadata("nullableComposableParameterAcrossModules.kt")
+    public void testNullableComposableParameterAcrossModules() throws Exception {
+        runTest("plugins/fir-plugin-prototype/testData/box/nullableComposableParameterAcrossModules.kt");
+    }
+
     @Test
     @TestMetadata("replaceActualFunctionBodyWitExpectDefaultValue.kt")
     public void testReplaceActualFunctionBodyWitExpectDefaultValue() throws Exception {