Always add a null function at table index zero

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

tests/compiler/declare.untouched.wat

@@ -7,6 +7,8 @@
  (import "my" "externalFunction" (func $declare/my.externalFunction))
  (import "my" "externalConstant" (global $declare/my.externalConstant i32))
  (global $HEAP_BASE i32 (i32.const 32))
+ (table 1 1 anyfunc)
+ (elem (i32.const 0) $null)
  (memory $0 1)
  (data (i32.const 8) "\n\00\00\00d\00e\00c\00l\00a\00r\00e\00.\00t\00s\00")
  (export "memory" (memory $0))
@@ -49,4 +51,6 @@
    )
   )
  )
+ (func $null (; 4 ;) (type $v)
+ )
 )