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/toStringOfInlineClassValue.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +InlineClasses
+
+// FILE: Z.kt
+inline class Z(val x: Int)
+
+// FILE: test.kt
+fun testZ(z: Z) = z.toString()
+fun testZT(z: Z) = "$z"
+fun testNZ(z: Z?) = z?.toString() // unboxed into Int after the null check
+fun testNZA(z: Z?) = z.toString() // calls Any?.toString() on boxed value
+fun testNZT(z: Z?) = "$z" // same
+
+// @TestKt.class:
+// 3 INVOKESTATIC Z\.toString-impl \(I\)Ljava/lang/String;
+// 2 INVOKESTATIC java/lang/String.valueOf