fix(cli): fix `commit.gpgsign` test in `omz pr test`

Since `set -e` is enabled, when `commit.gpgsign` is not set the
`git config` command would show an error. Given that it is technically
not ignored, the subshell would exit.

With this change, the `commit.gpgsign` setting is properly tested
by doing the fallback test if the command fails, so no exit status
code ends up quiting the subshell.