Add check if radars tuple is not empty

[Quba1]

This is the same PR as #990 (reopened as I have forgotten to add the check to the map_to_grid function).

In some edge cases (in my case when radars tuple is created in a loop) the empty tuple can be passed to the gates_to _grid or map_to_grid functions. In that casetuple index out of range error occures when radars[0].altitude['data'] is acessed, and it is not always obvious what is the cause of that error.

I propose to add a simple check if radars tuple length is equal to 0. If yes then ValueError exception is raised with a more meaningful error message indicating what the issue is.
The placement of that if statement is just after conversion of single Radar object into radars tuple, so that tuple length can be checked without worring about edge cases.
In this solution also length of gatefilters tuple is implicitly checked as gatefilters must have the same length as radars.
The check is also added to grid_from_radars function as there is no need to call the gridding functions if the radars tuple is empty.

[zssherman]

Thanks @Quba1! Everything looks good, down the road we may need a new unit tests to pytest raise the error. Merging.