Always add a null function at table index zero

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

tests/compiler/class-with-boolean-field.optimized.wat

@@ -1,5 +1,6 @@
 (module
  (type $i (func (result i32)))
+ (type $v (func))
  (memory $0 0)
  (export "memory" (memory $0))
  (export "test" (func $class-with-boolean-field/test))
@@ -12,4 +13,7 @@
    (i32.const 0)
   )
  )
+ (func $null (; 1 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )