[Engine] Return correct error value in sml_variable_get_value
[Engine] Return correct error value in sml_variable_get_value

If sml_variable_get_value fails, we should return a valid float value.
Using NAN.

Signed-off-by: Otavio Pontes <a04f37e3ccb281cf4119e4b2fa8286ed94121d92@intel.com>

solettaproject/soletta-machine-learning,anselmolsm/soletta-machine-learning,anselmolsm/soletta-machine-learning,solettaproject/soletta-machine-learning,solettaproject/soletta-machine-learning,anselmolsm/soletta-machine-learning