Fix incorrect "senseless null in when" on nullable type

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/noSenselessNullOnNullableType.kt

@@ -0,0 +1,21 @@
+// FILE: A.java
+
+class A {
+    enum Empty {}
+
+    static Empty foo() { return null; }
+}
+
+// FILE: 1.kt
+
+fun bar() = when (A.foo()) {
+    null -> "null"
+    else -> "else"
+}
+
+fun <T : Number?> baz(t: T) = when (t) {
+    is Int -> "int"
+    is Long -> "long"
+    null -> "null"
+    else -> "else"
+}

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/noSenselessNullOnNullableType.txt

@@ -0,0 +1,28 @@
+package
+
+internal fun bar(): kotlin.String
+internal fun </*0*/ T : kotlin.Number?> baz(/*0*/ t: T): kotlin.String
+
+public/*package*/ open class A {
+    public/*package*/ constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public/*package*/ final enum class Empty : kotlin.Enum<A.Empty!> {
+        public/*package*/ constructor Empty()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: A.Empty!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+        // Static members
+        public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): A.Empty
+        public final /*synthesized*/ fun values(): kotlin.Array<A.Empty>
+    }
+
+    // Static members
+    public/*package*/ open fun foo(): A.Empty!
+}