git: delete branches in `gbda` only if there are any (#6079)

It doesn't make sense to run `git branch -d $BRANCH` if the `$BRANCH` is empty.

plugins/git/git.plugin.zsh

@@ -50,7 +50,7 @@ alias gap='git apply'
 alias gb='git branch'
 alias gba='git branch -a'
 alias gbd='git branch -d'
-alias gbda='git branch --no-color --merged | command grep -vE "^(\*|\s*(master|develop|dev)\s*$)" | command xargs -n 1 git branch -d'
+alias gbda='git branch --no-color --merged | command grep -vE "^(\*|\s*(master|develop|dev)\s*$)" | command xargs -r -n 1 git branch -d'
 alias gbD='git branch -D'
 alias gbl='git blame -b -w'
 alias gbnm='git branch --no-merged'