fix(starship): keep `ZSH_THEME` if not installed (#12309)

plugins/starship/starship.plugin.zsh

@@ -1,7 +1,7 @@
-# ignore oh-my-zsh theme
-unset ZSH_THEME
-
 if (( $+commands[starship] )); then
+  # ignore oh-my-zsh theme
+  unset ZSH_THEME
+
   eval "$(starship init zsh)"
 else
   echo '[oh-my-zsh] starship not found, please install it from https://starship.rs'