Fix error reports on missing include paths

[ferd]

In some cases (nested includes?) paths end up in such a way that joining
them breaks up and hard-crashes rebar3. This patch specifically handles
this scenario to fix things by avoiding passing empty lists to
filename:join.

fixes #914.

I'm not sure how exactly to reproduce for tests. I showed it is fixed it by using https://github.com/rabbitmq/rabbitmq-erlang-client :

→ rebar3 compile
===> Verifying dependencies...
===> Compiling amqp_client
===> Compiling /home/ferd/code/experiment/rabbitmq-erlang-client/_build/default/lib/amqp_client/src/amqp_gen_consumer.erl failed
/home/ferd/code/experiment/rabbitmq-erlang-client/_build/default/lib/amqp_client/include/amqp_client.hrl:20: can't find include lib "rabbit_common/include/rabbit.hrl"
/home/ferd/code/experiment/rabbitmq-erlang-client/_build/default/lib/amqp_client/include/amqp_client.hrl:21: can't find include lib "rabbit_common/include/rabbit_framing.hrl"
...

[tsloughter]

Ah, good fix.