Fixed a crashing bug when printing xml trace information for weird rules, in which an operator is a string, for example in TestMatchTimeInterrupt.  Soar was attempting to access the "level: of the value element of an operator wme, which doesn't exist if the value is a string rather than an identifier.  This bug was masked prior to 9.4 because symbols were a union and this function was just producing trace ouput.  It was probably printing out garbage data but did not crash.  (Note:  I actually fixed this several versions ago, but I double-checked the logic of it and added comments explaining it in this commit.)