Do not force coercion to Unit for nullable lambda return type

#KT-41005 Fixed

compiler/testData/codegen/box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint.kt

@@ -0,0 +1,22 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// TARGET_BACKEND: JVM
+
+// FILE: TestJ.java
+
+public class TestJ {
+    public static <T> In<T> materialize() {
+        return null;
+    }
+}
+
+// FILE: test.kt
+
+class In<in T>
+
+fun <T> inferred(e: In<T>?, l: () -> T): T = l()
+
+fun box(): String {
+    // coercion to Unit for T!
+    val inferred = (inferred(TestJ.materialize<Unit>(), { null })).toString()
+    return if (inferred == "kotlin.Unit") "OK" else "fail : $inferred"
+}

compiler/testData/codegen/box/inference/noCoercionToUniForNullableLambdaReturnType.kt

@@ -0,0 +1,7 @@
+fun <T> nullableK(m: () -> T?) = m()
+
+fun box(): String {
+    // no coercion to Unit for T?
+    val nullableK = (nullableK<Unit> { null }).toString()
+    return if (nullableK == "null") "OK" else  "fail: $nullableK"
+}