Always add a null function at table index zero

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

tests/compiler/namespace.optimized.wat

@@ -19,4 +19,7 @@
    (call $namespace/Joined.anotherFunc)
   )
  )
+ (func $null (; 3 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )