Add empty checks

[bashbaug]

fixes #338 by explicitly passing nullptr when a vector is empty.

There were a lot of these... I think I got them all.

I added a bunch of unit tests when related tests existed, but there are still some unit testing gaps. We're no worse off than we were previously, though.

[bashbaug]

@jansol, mind taking a look?

[jansol]

Looks good, thanks!

For consistency the various calls to clCreateImageWithProperties and clCreateBufferWithProperties could be switched to using the ternary operator instead of two separate calls in an if-else clause, but functionally they are fine as is.

[jansol]

Isn't the idea that the in-memory representation of the wrapper class objects is == the underlying handles? From a quick look my understanding is that this should be the case for Device too.

[bashbaug]

That's the general idea, yes, but device objects are an exception since they have an extra variable:

template <>
class Wrapper<cl_device_id>
{
public:
    typedef cl_device_id cl_type;

protected:
    cl_type object_;
    bool referenceCountable_;
...

There might be a better way to do this, but I think it's outside of the scope of this PR...

[jansol]

Ah, I missed that 2nd field. Yea that's a whole separate topic then.

[bashbaug]

For consistency the various calls to clCreateImageWithProperties and clCreateBufferWithProperties could be switched to using the ternary operator instead of two separate calls in an if-else clause, but functionally they are fine as is.

May as well fix this while we're at it. Done!