Always add a null function at table index zero

This allows function references (a table index internally) to be nullable

tests/compiler/class-overloading.optimized.wat

@@ -17,4 +17,7 @@
    (i32.const 0)
   )
  )
+ (func $null (; 3 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )