Make 'when' on nullable enums exhaustive if 'null' entry is present

#KT-2902 Fixed

compiler/testData/diagnostics/tests/when/ElseOnNullableEnum.kt

@@ -0,0 +1,37 @@
+// KT-2902 Check for null should be required when match nullable enum element
+
+// FILE: 1.kt
+
+enum class E { A B }
+
+fun test(e: E?) = <!NO_ELSE_IN_WHEN!>when<!> (e) {
+    E.A -> 1
+    E.B -> 2
+}
+
+fun withNull(e: E?) = when (e) {
+    E.A -> 3
+    E.B -> 4
+    null -> null
+}
+
+fun nullableNothing(): Nothing? = null
+fun withNullableNothing(e: E?) = when (e) {
+    E.A -> 5
+    E.B -> 6
+    nullableNothing() -> null
+}
+
+fun platformType() = <!NO_ELSE_IN_WHEN!>when<!> (J.foo()) {
+    E.A -> 7
+    E.B -> 8
+}
+
+
+// FILE: J.java
+
+class J {
+    static E foo() {
+        return E.A;
+    }
+}