Always add a null function at table index zero

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

tests/compiler/external.optimized.wat

@@ -12,4 +12,7 @@
  (export "two" (func $external/two))
  (export "three" (func $external/three))
  (export "var_" (global $external/var_))
+ (func $null (; 4 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )