[LQDevice] Avoid nil values on Device attributes

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Tests] no longer run over Demo app target

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
Assert if API Token is empty

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Fix] a race condition on _loadedLiquidPackage

Solved on three methods:

* `loadLiquidPackageSynced:`
* `track:attributes:allowLqdEvents:`
* `invalidateTargetThatIncludesVariable:`

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Tests] to avoid race conditions with lots of threads

* 1200 threads in simultaneous

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>