fix(random): fix `ZSH_THEME_RANDOM_QUIET` check (#10534)

BREAKING CHANGE: For consistency, `ZSH_THEME_RANDOM_QUIET` now needs to
be `true` if you want to silence the "Random theme loaded" message in the
`random` theme.

The wiki specified that 1 or true was valid, while the code just
checked whether it was set to any value. Being more strict makes
sure that we're consistent with the rest of the Settings.

Fixes #10534

themes/random.zsh-theme

@@ -42,6 +42,6 @@ else
   return 1
 fi
 
-if [[ -z "$ZSH_THEME_RANDOM_QUIET" ]]; then
+if [[ "$ZSH_THEME_RANDOM_QUIET" = true ]]; then
   echo "[oh-my-zsh] Random theme '${RANDOM_THEME}' loaded"
 fi