Always add a null function at table index zero

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

tests/compiler/switch.untouched.wat

@@ -4,6 +4,8 @@
  (type $v (func))
  (import "env" "abort" (func $~lib/env/abort (param i32 i32 i32 i32)))
  (global $HEAP_BASE i32 (i32.const 32))
+ (table 1 1 anyfunc)
+ (elem (i32.const 0) $null)
  (memory $0 1)
  (data (i32.const 8) "\t\00\00\00s\00w\00i\00t\00c\00h\00.\00t\00s\00")
  (export "memory" (memory $0))
@@ -773,4 +775,6 @@
    )
   )
  )
+ (func $null (; 9 ;) (type $v)
+ )
 )