Always add a null function at table index zero

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

tests/compiler/overflow.optimized.wat

@@ -1,5 +1,9 @@
 (module
+ (type $v (func))
  (memory $0 1)
  (data (i32.const 8) "\0b\00\00\00o\00v\00e\00r\00f\00l\00o\00w\00.\00t\00s")
  (export "memory" (memory $0))
+ (func $start (; 0 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )