Fix StackValue.coerce void to Object

We can only put Unit.VALUE on stack if we're requested an instance of Object or
Unit. In other cases (arbitrary objects / arrays) we put null instead

compiler/testData/codegen/box/functions/coerceVoidToArray.kt

@@ -0,0 +1,16 @@
+fun a(): IntArray? = null
+
+fun b() = throw Exception()
+
+fun foo(): IntArray = a() ?: b()
+
+
+fun box(): String {
+    try {
+        foo()
+    } catch (e: Exception) {
+        return "OK"
+    }
+
+    return "Fail"
+}

compiler/testData/codegen/box/functions/coerceVoidToObject.kt

@@ -0,0 +1,16 @@
+fun a(): String? = null
+
+fun b() = throw Exception()
+
+fun foo(): String = a() ?: b()
+
+
+fun box(): String {
+    try {
+        foo()
+    } catch (e: Exception) {
+        return "OK"
+    }
+
+    return "Fail"
+}