T5543: IGMP: fix source address handling in static joins

The following command expects to join source-specific multicast group 239.1.2.3
on interface eth0, where the source address is 192.0.2.1.

set protocols igmp interface eth0 join 239.1.2.3 source 192.0.2.1

This command should generate FRR config:

interface eth0
ip igmp
ip igmp join 239.1.2.3 192.0.2.1
exit

However, there is a bug in the Jinja template where if ifaces[iface].gr_join[group]
is mostly evaluated as false because iface is a loop variable from another loop.