Fix error when no backends are available.
Fix error when no backends are available.

umar456/arrayfire,9prady9/arrayfire,arrayfire/arrayfire,arrayfire/arrayfire,umar456/arrayfire,9prady9/arrayfire,umar456/arrayfire,umar456/arrayfire,9prady9/arrayfire,9prady9/arrayfire,arrayfire/arrayfire,arrayfire/arrayfire