gluon-core: dont fail on unset interface role (#3095)

In case no interface role was selected for a given interface,
gluon-reconfigure currently fails with

Configuring: 021-interface-roles
/usr/bin/lua: /usr/lib/lua/gluon/util.lua:25: bad argument #1 to 'pairs' (table expected, got nil)
stack traceback:
	[C]: in function '?'
	/usr/lib/lua/simple-uci.lua:22: in function 'foreach'
	./021-interface-roles:47: in main chunk
	[C]: ?

Check we actually have a role set before accessing the table.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit c4d37036)