fix(virtualenv): allow empty prefix/suffix in prompt function (#9763)

Fixes #9763

plugins/virtualenv/virtualenv.plugin.zsh

 function virtualenv_prompt_info(){
   [[ -n ${VIRTUAL_ENV} ]] || return
-  echo "${ZSH_THEME_VIRTUALENV_PREFIX:=[}${VIRTUAL_ENV:t}${ZSH_THEME_VIRTUALENV_SUFFIX:=]}"
+  echo "${ZSH_THEME_VIRTUALENV_PREFIX=[}${VIRTUAL_ENV:t}${ZSH_THEME_VIRTUALENV_SUFFIX=]}"
 }
 
 # disables prompt mangling in virtual_env/bin/activate