Don't recognize nullable function types as inlinable

#KT-19679 Fixed

compiler/testData/codegen/boxInline/defaultValues/maskElimination/kt19679_2.kt

@@ -0,0 +1,15 @@
+// FILE: 1.kt
+package test
+
+inline fun build(noinline pathFunc: (() -> String)? = null) {
+    pathFunc?.invoke()
+}
+
+// FILE: 2.kt
+
+import test.*
+
+fun box(): String {
+    build ()
+    return "OK"
+}