interfaces: handle missing DHCPv6 PD size

[jpawlowski]

TL;DR

Use a null fallback when checking an optional DHCPv6 prefix delegation size.

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Describe the problem

Tracked in #10764.

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Describe the proposed solution

Apply a null fallback to both numeric prefix delegation checks. Existing numeric values remain unchanged, while an absent value follows the existing nonnumeric path.

Tested on OPNsense 26.7.2_2 with a DHCPv6 configuration lacking the PD-size field. IA-NA output was preserved, IA-PD remained omitted, and no PHP errors were added. PHP syntax validation also passed.

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Related issue

Fixes #10764

[fichtner]

Isn’t PD size mandatory?

[jpawlowski]

I ended up with this setting when I was not finishing the initial setup wizard because the machine has only one network interface. so apparently one can end up in such situation.

[fichtner]

Thanks. Need to check this. Historically the wizard does not cover IPv6 but it also should not delete settings. Not saying that it did, but better to check that.

[jpawlowski]

I should mention that I was also disabling IPv6 later because it had connection issues. I wasn't investigating those so far, maybe later. But maybe that's when this error was produced. I will start a new VM from scratch when there it time, also to help takle the IPv6 connectivity issue.