FIR: allow no else branch for when on Nothing(?)

compiler/testData/diagnostics/tests/when/whenOnNothing.fir.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNREACHABLE_CODE
+
+// exhaustive
+fun test1(n: Nothing) = when (n) { }
+fun test2(n: Nothing?) = when (n) {
+    null -> {}
+}
+
+// not exhaustive
+fun test3(n: Nothing?) = <!NO_ELSE_IN_WHEN!>when<!> (n) {
+}

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

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNREACHABLE_CODE
+
+// exhaustive
+fun test1(n: Nothing) = when (n) { }
+fun test2(n: Nothing?) = <!NO_ELSE_IN_WHEN!>when<!> (<!DEBUG_INFO_CONSTANT!>n<!>) {
+    null -> {}
+}
+
+// not exhaustive
+fun test3(n: Nothing?) = <!NO_ELSE_IN_WHEN!>when<!> (<!DEBUG_INFO_CONSTANT!>n<!>) {
+}

compiler/testData/diagnostics/tests/when/whenOnNothing.txt

@@ -0,0 +1,3 @@
+package
+
+public fun foo(/*0*/ n: kotlin.Nothing, /*1*/ nn: kotlin.Nothing?): kotlin.Unit