Always add a null function at table index zero

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

tests/compiler/assert.optimized.wat

@@ -1,6 +1,10 @@
 (module
+ (type $v (func))
  (memory $0 1)
  (data (i32.const 8) "\t\00\00\00a\00s\00s\00e\00r\00t\00.\00t\00s")
  (data (i32.const 32) "\0c\00\00\00m\00u\00s\00t\00 \00b\00e\00 \00t\00r\00u\00e")
  (export "memory" (memory $0))
+ (func $start (; 0 ;) (; has Stack IR ;) (type $v)
+  (nop)
+ )
 )