Check for empty plist in backward problem.  Fixes #287
Check for empty plist in backward problem.  Fixes #287

FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI