net/haproxy: Allow to have no DNS servers defined in SYSTEM: SETTINGS: GENERAL -> Networking

[cookiemonsteruk]

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Is your feature request related to a problem? Please describe.
Today I upgraded the firewall from 23.7 to 24.7 with 24.1 as first step. Reviewing the services I noticed a problem with HAProxy. I remembered seeing something similar.
From https://forum.opnsense.org/index.php?topic=39484.0
It seems that from OPN 24.1 onwards, if in SYSTEM: SETTINGS: GENERAL -> Networking there is no DNS server defined, HAProxy plugin syntax parsing will fail with the following errors:

HAProxy configtest found critical errors
[NOTICE] (58508) : haproxy version is 2.8.10-f28885f
[NOTICE] (58508) : path to executable is /usr/local/sbin/haproxy
[ALERT] (58508) : config : Proxy '<OCSP-UPDATE>': Can't find resolvers section 'default' for do-resolve action.
[ALERT] (58508) : config : Proxy '<HTTPCLIENT>': Can't find resolvers section 'default' for do-resolve action.
[ALERT] (58508) : config : Fatal errors found in configuration.

However I think the service does go ahead and start. After all these are alerts. My guess is that a new version of HAProxy shipped with new requirements.
That said we also have the message in the plugin's settings:
There are pending configuration changes that must be applied in order for them to take effect. To review them visit the [Config Diff](https://192.168.5.1:55443/ui/haproxy/export#diff) page.
That diff shows:

--- /usr/local/etc/haproxy.conf	2024-07-28 03:48:10.914848000 +0100
+++ /usr/local/etc/haproxy.conf.staging	2024-08-06 16:33:40.105273000 +0100
@@ -13,6 +13,9 @@
     hard-stop-after             60s
     no strict-limits
     maxconn                     10
+    tune.ssl.ocsp-update.mindelay 300
+    tune.ssl.ocsp-update.maxdelay 3600
+    httpclient.resolvers.prefer   ipv4
     tune.ssl.default-dh-param   2048

I have not applied the dfiff.

Describe the solution you'd like
I would like the plugin to satisfy the requirement of the service so we have a clean systax tester and we can apply any required diffs of changes without worry of breakage.

Describe alternatives you've considered
N/A

Additional context
N/A