Always add a null function at table index zero

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

tests/compiler/unary.optimized.wat

@@ -347,4 +347,7 @@
    (get_local $3)
   )
  )
+ (func $null (; 1 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )