Make transaction view tolerate missing listings

this is done bit quickly, so thorough review is welcome, and comments about if this kind of quick fix approach makes sense now?
Anyway would be great that the transaction view doesn't break like it does now, and it will take a while before we get to improve the delete user functionality, so meanwhile these kind of changes could do the job, and not endanger normal transactions where listing and author are not gone. 
Should always return symbol, not a placeholder string. Does this break anything downstream like looking up a css class based on known symbols or something?