fish_greeting: show private mode message if $fish_greeting is empty list

This was droped because of Cartesian product expansion.
Also fix the spurious dot in case the greeting is "".

Fixes #7974

CHANGELOG.rst

@@ -41,6 +41,7 @@ Interactive improvements
 - ``funced`` won't include an entry on where a function is defined, thanks to the new ``functions --no-details`` option (:issue:`7879`).
 - The git prompt now has the same symbol order in normal and "informative" mode, and it's customizable via ``$__fish_git_prompt_status_order`` (:issue:`7926`).
 - Variable ``fish_killring`` containing entries from killring is now available (:issue:`7445`).
+- ``fish --private`` prints a note on private mode on startup even if ``$fish_greeting`` is an empty list (:issue:`7974`).
 
 New or improved bindings
 ^^^^^^^^^^^^^^^^^^^^^^^^

share/functions/fish_greeting.fish

@@ -7,7 +7,11 @@ function fish_greeting
 
     if set -q fish_private_mode
         set -l line (_ "fish is running in private mode, history will not be persisted.")
-        set -g fish_greeting $fish_greeting.\n$line
+        if set -q fish_greeting[1]
+            set -g fish_greeting $fish_greeting\n$line
+        else
+            set -g fish_greeting $line
+        end
     end
 
     # The greeting used to be skipped when fish_greeting was empty (not just undefined)