plugin-caller: make no-ops always successful

plugin-caller: make no-ops always successful

proxmox-acme is used to call the dns-plugins from acme.sh and has the
config editing (saving/clearing) turned int no-ops.

bash's `return` statement w/o argument returns the value of the last
executed command (the one before our no-op method was called) (see
`help return` in bash).
This leads to unexpected behavior in some plugins, which call one of
the methods as last statement join the next step with `&&`.

tested bash behavior with:
```
foo() { return; }; if [ -z 'x' ]; then :; else foo ; fi; echo $?
```

reported in our community-forum:
https://forum.proxmox.com/threads/pmg-acme-dns-with-cyon-failing-to-issue-certificate.92762

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>