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/bytecodeText/inlineClasses/toStringOfReferenceInlineClassValue.kt

@@ -0,0 +1,17 @@
+// !LANGUAGE: +InlineClasses
+// Completely incorrect bytecode - see `box/inlineClasses/toStringOfUnboxedNullable.kt`
+// IGNORE_BACKEND: JVM
+
+// FILE: Z.kt
+inline class Z(val x: Any)
+
+// FILE: test.kt
+fun testZ(z: Z) = z.toString()
+fun testZT(z: Z) = "$z"
+fun testNZ(z: Z?) = z?.toString() // `Z?` is unboxed into `Any?` even before the null check
+fun testNZA(z: Z?) = z.toString() // so all of these call toString-impl
+fun testNZT(z: Z?) = "$z"
+
+// @TestKt.class:
+// 5 INVOKESTATIC Z\.toString-impl \(Ljava/lang/Object;\)Ljava/lang/String;
+// 0 INVOKESTATIC java/lang/String.valueOf