Fix schema: allow empty optional interface/firewall values

Optional match()?/list()? validators reject empty string, which is the
default value, blocking add-on start. Use regexes that accept empty OR valid.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

netmaker/config.yaml

@@ -40,7 +40,7 @@ schema:
   log_level: list(trace|debug|info|notice|warning|error|fatal)
   host_name: str?
   port: int(0,65535)?
-  interface: "match(^netmaker[a-zA-Z0-9_-]*$)?"
+  interface: "match(^(?:netmaker[a-zA-Z0-9_-]*)?$)?"
   endpoint: str?
   endpoint6: str?
-  firewall: list(iptables|nftables)?
+  firewall: "match(^(?:iptables|nftables)?$)?"