Always add a null function at table index zero

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

tests/compiler/empty.optimized.wat

@@ -1,4 +1,8 @@
 (module
+ (type $v (func))
  (memory $0 0)
  (export "memory" (memory $0))
+ (func $null (; 0 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )