Minor addition to variable comments to clarify use of %g
Minor style improvement
Whitespace fix
Support empty vpn_ip4 [1/3]

This change supports env-var option processing without a VPN IP.
Support empty vpn_ip4 [2/3]

Handle session updates with empty vpn_ip4 %o val
Support empty vpn_ip4 [3/3]

Update the SQL schemas to support an empty vpn_ip4 for the session.
Support empty vpn_ip4 [0/3]

OpenVPN may not necessarily issue an IP to the client during connection,
such as when configured without a pool, the pool is exhausted, or in
some bridging setups.

These changes update the env-var processing, update codepaths, and SQL
schemas to allow for this.