Revert "agnoster: fix VIRTUAL_ENV_DISABLE_PROMPT logic (#8050)" (#8061)

This reverts commit 59c1ec80aa9b018db34078e69a27e57aea5ad761.

themes/agnoster.zsh-theme

@@ -204,7 +204,7 @@ prompt_dir() {
 # Virtualenv: current working virtualenv
 prompt_virtualenv() {
   local virtualenv_path="$VIRTUAL_ENV"
-  if [[ -n $virtualenv_path && -z $VIRTUAL_ENV_DISABLE_PROMPT ]]; then
+  if [[ -n $virtualenv_path && -n $VIRTUAL_ENV_DISABLE_PROMPT ]]; then
     prompt_segment blue black "(`basename $virtualenv_path`)"
   fi
 }