[zsh] Move the hello message to its own Darwin-specific say_hello function

zsh/func/init_profile_darwin

@@ -5,11 +5,5 @@ if ssh-add --apple-load-keychain; then
     echo
 fi
 
-if binary_exists sw_vers; then
-    cat_program=cat
-    if binary_exists lolcat; then
-        cat_program=lolcat
-    fi
-    print "It's `date +'%H:%M on %A, %B %d'`.\nThis machine is a $HWMODEL.\nYou're running macOS $OSVERSION.\n" | $cat_program
-fi
-
+autoload -Uz init_profile_darwin_say_hello
+init_profile_darwin_say_hello

zsh/func/init_profile_darwin_say_hello

@@ -0,0 +1,13 @@
+#!/usr/bin/env zsh
+# Eryn Wells <eryn@erynwells.me>
+
+if binary_exists sw_vers && [[ -n "$HWMODEL" && -n "$OSVERSION" ]]; then
+    cat_program=cat
+    if binary_exists lolcat; then
+        cat_program=lolcat
+    fi
+    print "It's `date +'%H:%M on %A, %B %d'`.\nThis machine is a $HWMODEL.\nYou're running macOS $OSVERSION.\n" | $cat_program
+    return 0
+fi
+
+return 1