Make emptyMap upperbound to be Any? rather than Any.

#KT-9963

libraries/stdlib/src/kotlin/collections/Maps.kt

@@ -6,7 +6,7 @@ package kotlin
 import java.io.Serializable
 import java.util.*
 
-private object EmptyMap : Map<Any, Nothing>, Serializable {
+private object EmptyMap : Map<Any?, Nothing>, Serializable {
     override fun equals(other: Any?): Boolean = other is Map<*,*> && other.isEmpty()
     override fun hashCode(): Int = 0
     override fun toString(): String = "{}"
@@ -14,11 +14,11 @@ private object EmptyMap : Map<Any, Nothing>, Serializable {
     override val size: Int get() = 0
     override fun isEmpty(): Boolean = true
 
-    override fun containsKey(key: Any): Boolean = false
+    override fun containsKey(key: Any?): Boolean = false
     override fun containsValue(value: Nothing): Boolean = false
-    override fun get(key: Any): Nothing? = null
-    override val entries: Set<Map.Entry<Any, Nothing>> get() = EmptySet
-    override val keys: Set<Any> get() = EmptySet
+    override fun get(key: Any?): Nothing? = null
+    override val entries: Set<Map.Entry<Any?, Nothing>> get() = EmptySet
+    override val keys: Set<Any?> get() = EmptySet
     override val values: Collection<Nothing> get() = EmptyList
 
     private fun readResolve(): Any = EmptyMap

libraries/stdlib/test/collections/CollectionBehaviors.kt

@@ -89,6 +89,10 @@ public fun <K, V> CompareContext<Map<K, V>>.mapBehavior() {
     if (expected.isEmpty().not())
         propertyEquals { contains(keySet().first()) }
 
+    propertyEquals { containsKeyRaw(keys.firstOrNull()) }
+    propertyEquals { containsValueRaw(values.firstOrNull()) }
+    propertyEquals { getRaw(null) }
+
     compareProperty( { keySet() }, { setBehavior("keySet") } )
     compareProperty( { entrySet() }, { setBehavior("entrySet") } )
     compareProperty( { values() }, { collectionBehavior("values") })