Always add a null function at table index zero

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

tests/compiler/class-extends.untouched.wat

@@ -1,6 +1,9 @@
 (module
  (type $iv (func (param i32)))
+ (type $v (func))
  (global $HEAP_BASE i32 (i32.const 8))
+ (table 1 1 anyfunc)
+ (elem (i32.const 0) $null)
  (memory $0 0)
  (export "memory" (memory $0))
  (export "test" (func $class-extends/test))
@@ -24,4 +27,6 @@
    (i32.const 3)
   )
  )
+ (func $null (; 1 ;) (type $v)
+ )
 )