[connman-qt] Fix null pointer reference to technology object

If connman is not present, NetworkTechnology object won't have a
properly initialized NetConnmanTechnologyInterface member. Make
objPath() to return an empty string, and setPowered() and scan() no-op
in this case to avoid crashing.
Merge pull request #140 from nemomobile/fix-technology-null-pointer-crash

[connman-qt] Fix null pointer reference to technology object