KT-13521 Warning right part of "expression ?: null" is useless

#KT-13521 Fixed

compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.kt

@@ -0,0 +1,20 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+// FILE: J.java
+
+import org.jetbrains.annotations.*;
+
+public class J {
+    @Nullable
+    public static J staticN;
+}
+
+// FILE: k.kt
+
+fun test() {
+    val a = J.staticN <!USELESS_ELVIS_RIGHT_IS_NULL!>?: null<!>
+    foo(a)
+}
+
+fun foo(a: Any?) {
+}

compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.txt

@@ -0,0 +1,14 @@
+package
+
+public fun foo(/*0*/ a: kotlin.Any?): kotlin.Unit
+public fun test(): kotlin.Unit
+
+public open class J {
+    public constructor J()
+    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
+
+    // Static members
+    @org.jetbrains.annotations.Nullable() public final var staticN: J?
+}