Authorize empty value for dhcp-option to disabled it

[LucasBoisserie]

Authorize empty value for dhcp-option to disabled it

Description

By default dnsmasq with dhcp enabled send standard options to DHCP clients (DNS server, default route, …)
To disabled it, we need to setup dnsmasq.con with dhcp-options with empty value which is not possible currently ( single , is always added)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.