185091247 - Placeholders are shown for null/empty operands

[dennisrcao]

WIP Do not review

[arnog]

FYI, you might want to consider 1815. The idea is to have an option so that empty groups would be made more prominent (given some width, and a background color). An empty group being different from a placeholder, as you can have valid content with empty groups (i.e. "2^{}") which doesn't have a placeholder.