How to avoid an undesired "nil" in defmacro

When writing a macro one often constructs the form to be evaluated, henceforth referred to as FORM, using the back-quote list construct.

So suppose that, in the middle of this process, one is about to add an element to FORM (typically another form), henceforth referred to as SUB-FORM, which, depending on a certain CONDITION (presumably a test based on some macro parameter), may perhaps make the addition of this SUB-FORM unnecessary. My gut instinct is to write

(defmacro NAME (VARS)
  `(...   ,(if CONDITION SUB-FORM)   ...)

As a result, when CONDITION fails, a "nil" is added to FORM. This is actually no big deal since, when FORM goes through eval-time, this undesired nil is evaluated, but this is not likely to be relevant in the finall result.

Still (perhaps being a bit of a purist) the presence of this unnecessary nil annoys me and my question has to do with possible ways to eliminate it.

I can of course think of lots of ways to do it, but I'd like to know if anyone has though of a specially elegant solution such as a variation of the function if, say if2, such that the construct

,(if2 CONDITION SUB-FORM)

would leave no trace in FORM upon the failure of CONDITION.

Here is a toy example illustrating the presence of this unwanted nil.

(defmacro hello-world (good-mood)
 `(progn
   (princ "Hello")
   ,(if good-mood '(princ " wonderful"))  ;; Here is the test involving the SUB-FORM (princ " cruel")
   (princ " world")))

When in bad mood, FORM, namely

(macroexpand '(hello-world nil))

becomes

(progn (princ "Hello") nil (princ " world"))

and you can see the innocuous but undesired nil showing up.

PS: In order to save your time let me say that I have already thought of solutions along the lines of:

1. Write hello-world using a defun rather than a defmacro.

2. Use append.

3. Delete nil from FORM before the defmacro returns it.

4. Stay in good-mood permanently :-)