Always add a null function at table index zero

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

tests/compiler/std/hash.untouched.wat

@@ -13,6 +13,8 @@
  (global $Infinity f64 (f64.const inf))
  (global $NaN f64 (f64.const nan:0x8000000000000))
  (global $HEAP_BASE i32 (i32.const 44))
+ (table 1 1 anyfunc)
+ (elem (i32.const 0) $null)
  (memory $0 1)
  (data (i32.const 8) "\00\00\00\00")
  (data (i32.const 16) "\01\00\00\00a\00")
@@ -536,4 +538,6 @@
    )
   )
  )
+ (func $null (; 5 ;) (type $v)
+ )
 )