Don't recognize nullable function types as inlinable

#KT-19679 Fixed

compiler/testData/codegen/boxInline/defaultValues/maskElimination/kt19679_3.kt

@@ -0,0 +1,20 @@
+// FILE: 1.kt
+package test
+
+@Suppress("NULLABLE_INLINE_PARAMETER")
+inline fun build(func: () -> Unit, pathFunc: (() -> String)? = null) {
+    func()
+
+    pathFunc?.invoke()
+}
+
+// FILE: 2.kt
+
+import test.*
+
+fun box(): String {
+    var result = "fail"
+    build ({ result = "OK" })
+
+    return result
+}