Fix warnings during compilation

[corteccia]

During building examples in Navio2/C++, regardless of cross-compilation or native build, some warnings appear:

Navio2/C++/Examples/AHRS.cpp: In function ‘std::string get_sensor_name(int, char**)’:
Navio2/C++/Examples/AHRS.cpp:414:1: warning: control reaches end of non-void function [-Wreturn-type]
      414 | }
          | ^
Navio2/C++/Examples/AccelGyroMag.cpp: In function ‘std::string get_sensor_name(int, char**)’:
Navio2/C++/Examples/AccelGyroMag.cpp:79:1: warning: control reaches end of non-void function [-Wreturn-type]
       79 | }
          | ^

It's needed to return string value in those functions even if control never reaches there. In the main function, if 'get_sensor_name' function returns an empty value, it will return EXIT_FAILURE which is why it is needed to return an empty string in case it's not Navio2.