Add inspection for when with only else (#1206)

Fixes #KT-12523

idea/testData/inspectionsLocal/whenWithOnlyElse/hasOtherBranches.kt

@@ -0,0 +1,11 @@
+// PROBLEM: none
+
+fun println(s: String) {}
+
+fun foo(a: Boolean, b: Boolean) {
+    <caret>when ("") {
+        "a" -> println("a")
+        else -> println("else")
+
+    }
+}