T2345: After examining comments changed by batch conversion of NULL

to nullptr, I've corrected a small number where it seemed clear
that the previous NULL should have been either "NULL" (where the
code is looking for a string with that value),  or null char / null byte
(where the comment is talking about the char/byte that terminates
a c-style string).

Also changed 6 places where the batch change had changed 'NULL ptr'
to 'nullptr ptr'. Now they simply say 'nullptr'.

src/USER-MISC/pair_tersoff_table.cpp

@@ -755,7 +755,7 @@ void PairTersoffTable::coeff(int narg, char **arg)
     error->all(FLERR,"Incorrect args for pair coefficients");
 
   // read args that map atom types to elements in potential file
-  // map[i] = which element the Ith atom type is, -1 if nullptr
+  // map[i] = which element the Ith atom type is, -1 if "NULL"
   // nelements = # of unique elements
   // elements = list of element names