Always add a null function at table index zero

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

tests/compiler/std/trace.optimized.wat

@@ -101,4 +101,7 @@
    (f64.const 5.5)
   )
  )
+ (func $null (; 3 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )