failure when BUILD_RESTRICT_LANG is not defined.

[rvalyi]

not assigning BUILD_RESTRICT_LANG in the Dockerfile leads to build errors such as:

find: paths must precede expression: `f'
remote: The command '/bin/sh -c build-odoo' returned a non-zero code: 1
remote: 

This is because if [[ $BUILD_RESTRICT_LANG ]] is not a correct Bash test. The correct way should be something like
if [[ ! -z $BUILD_RESTRICT_LANG ]]