FIC: add codegen tests, adapt previously tests for SAMs

compiler/testData/codegen/box/funInterface/nullableSam.kt

@@ -0,0 +1,24 @@
+// !LANGUAGE: +NewInference +FunctionInterfaceConversion +SamConversionPerArgument +SamConversionForKotlinFunctions
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+fun interface KRunnable {
+    fun invoke()
+}
+
+fun isNull(r: KRunnable?): Boolean {
+    if (r == null) return true
+    r.invoke()
+    return false
+}
+
+fun nullableFun(fromNull: Boolean): (() -> Unit)? =
+    if (fromNull) null else {{}}
+
+fun box(): String {
+    if (!isNull(nullableFun(true))) return "Fail 1"
+    if (isNull(nullableFun(false))) return "Fail 2"
+    if (!isNull(null)) return "Fail 3"
+    if (isNull {}) return "Fail 4"
+    return "OK"
+}