Always add a null function at table index zero

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

tests/compiler/import.untouched.wat

@@ -5,6 +5,8 @@
  (global $export/b i32 (i32.const 2))
  (global $export/c i32 (i32.const 3))
  (global $HEAP_BASE i32 (i32.const 8))
+ (table 1 1 anyfunc)
+ (elem (i32.const 0) $null)
  (memory $0 0)
  (export "memory" (memory $0))
  (start $start)
@@ -69,4 +71,6 @@
   )
   (call $export/ns.two)
  )
+ (func $null (; 5 ;) (type $v)
+ )
 )