Always add a null function at table index zero

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

tests/compiler/for.untouched.wat

@@ -4,6 +4,8 @@
  (import "env" "abort" (func $~lib/env/abort (param i32 i32 i32 i32)))
  (global $for/i (mut i32) (i32.const 0))
  (global $HEAP_BASE i32 (i32.const 24))
+ (table 1 1 anyfunc)
+ (elem (i32.const 0) $null)
  (memory $0 1)
  (data (i32.const 8) "\06\00\00\00f\00o\00r\00.\00t\00s\00")
  (export "memory" (memory $0))
@@ -257,4 +259,6 @@
    )
   )
  )
+ (func $null (; 2 ;) (type $v)
+ )
 )