If a null input is given to the btk::MergeAcquistionFilter, a warning message is now displayed.
If a null input is given to the btk::MergeAcquistionFilter, a warning message is now displayed.

letaureau/b-tk.core,Biomechanical-ToolKit/BTKCore,xyproto/b-tk.core,Biomechanical-ToolKit/BTKCore,letaureau/b-tk.core,Biomechanical-ToolKit/BTKCore,xyproto/b-tk.core,xyproto/b-tk.core,xyproto/b-tk.core,letaureau/b-tk.core,letaureau/b-tk.core,letaureau/b-tk.core,Biomechanical-ToolKit/BTKCore,letaureau/b-tk.core,Biomechanical-ToolKit/BTKCore,xyproto/b-tk.core,xyproto/b-tk.core,Biomechanical-ToolKit/BTKCore,letaureau/b-tk.core,letaureau/b-tk.core,Biomechanical-ToolKit/BTKCore,xyproto/b-tk.core,xyproto/b-tk.core,Biomechanical-ToolKit/BTKCore