Prevent accidentally zeroing last_src in image_questions

[#60268936] Retaking a snapshot copies `current_src` into `last_src` twice due to different method calls. After the first time, `current_src` is set to ''. This meant the second time, last_src became ''. That prevented the snapshot retake from being properly canceled. This change checks to see if `current_src` is an empty string before allowing it to be copied into `last_src`.

https://www.pivotaltracker.com/story/show/60268936
Cleanup on aisle javascript