virtualenv: Handle not being in a virtualenv

Pre-Poetry support, we only entered this codepath when we were inside a
virtualenv. Now we enter it for every directory, but we don't want to
display an empty segment for non-virtualenv locations.

internal/p10k.zsh

@@ -4320,6 +4320,10 @@ prompt_virtualenv() {
       _virtualenv_load_name_poetry $idx
     fi
   fi
+  # Display nothing if there's no active virtualenv
+  if [[ -z $_virtualenv_name ]]; then
+    return
+  fi
   msg+="$_POWERLEVEL9K_VIRTUALENV_LEFT_DELIMITER${_virtualenv_name//\%/%%}$_POWERLEVEL9K_VIRTUALENV_RIGHT_DELIMITER"
   case $_POWERLEVEL9K_VIRTUALENV_SHOW_WITH_PYENV in
     false)