Properties serializer does not handle map null value correctly.

[devsvc]

Describe the bug
Properties serializer does not handle map null value correctly. Properties.encodeToMap serialized the key without value. So the map it encoded could not be decoded.

To Reproduce

class TestSer {

    @Serializable
    data class Data(
        val m: Map<String, String?> = mapOf()
    )

    @Test
    fun test() {
        val data = Data(mapOf(
            "a" to "1",
            "b" to null
        ))

        val map = Properties.encodeToMap(data)
        println(map)
        val data1: Data = Properties.decodeFromMap(map)
        println(data1)
    }

}

{m.0=a, m.1=1, m.2=b}

Expected behavior
map should be {m.0=a, m.1=1, m.2=b, m.3=null}

if it produce {m.a=1, m.b=null} will be much better

Environment

• Kotlin version: [1.5.20]
• Library version: [1.2.2]
• Kotlin platforms: [JVM]
• IDE version (if bug is related to the IDE) [Intellij Comunity 2021.2]

[sandwwraith]

It can't return map with nulls because encodeToMap has return type Map<String, Any>. I believe this was done according to Java's Properties behavior

[sandwwraith]

But it should be indeed fixed: dangling key shouldn't be present in map

[mkotsbak]

My feature request #1823 would probably solve this case.

[Sage-Kreiter]

Any update on this?

[sandwwraith]

Not as far as I know