Add test for KT-41254

It was fixed along with #KT-41005

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

@@ -0,0 +1,14 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+
+class Inv<T>(val x: T?)
+
+fun <R> foo(f: () -> R?): Inv<R> {
+    val r = f()
+    if (r != null) throw Exception("fail, result is not null: $r")
+    return Inv(r)
+}
+
+fun box(): String {
+    val r: Inv<Unit> = foo { if (false) Unit else null }
+    return "OK"
+}