JVM_IR: check for null when converting unboxed inline classes to strings

`C?` can be unboxed into `T?` if if wraps a reference type `T!!`, but in
this case `null` is not a valid value to pass to `toString-impl`.

 #KT-42005 Fixed

compiler/testData/codegen/box/inlineClasses/toStringOfUnboxedNullable.kt

@@ -0,0 +1,21 @@
+// !LANGUAGE: +InlineClasses
+// IGNORE_BACKEND: JVM
+// IGNORE_LIGHT_ANALYSIS
+inline class IC(val x: String)
+
+fun IC?.foo() = toString() // `IC?` unboxed into `String?`
+fun IC?.bar() = "$this"
+
+fun assertEquals(a: String, b: String) {
+    if (a != b) throw AssertionError("$a != $b")
+}
+
+fun box(): String {
+    assertEquals((null as IC?).foo(), "null")
+    assertEquals((null as IC?).foo(), (null as IC?).toString())
+    assertEquals((null as IC?).foo(), (null as IC?).bar())
+    assertEquals(IC("x").foo(), "IC(x=x)")
+    assertEquals(IC("x").foo(), IC("x").toString())
+    assertEquals(IC("x").foo(), IC("x").bar())
+    return "OK"
+}