Remove invariant on full_lhs_value in printing goto_tracet

This invariant in the printing is preventing other, otherwise usefull
output from being printed, in the case where a goto trace has been
produced which contains a `full_lhs_value` set to `nil` in an assignment
step.

Author: thomasspriggs

src/goto-programs/json_goto_trace.cpp

@@ -132,9 +132,6 @@ void convert_decl(
   }
   else
   {
-    DATA_INVARIANT(
-      step.full_lhs_value.is_not_nil(),
-      "full_lhs_value in assignment must not be nil");
     full_lhs_value = json(step.full_lhs_value, ns, ID_unknown);
   }