Pixel aspect ratio on Eyeon Fusion

Reading the pixel aspect ratio could create an error on Eyeon Fusion.
According to the OpenFX documentation the pixel aspect property should
exist on Clips. If there is an error we assume that PAR is 1...

Issue #254
tuttlePlugin: propGetString could return NULL

If there is no string content, some hosts could set nothing into the
pointer instead of "". So the pointer could stay NULL.

Issue #254