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/run.cpp

@@ -91,7 +91,7 @@ void Run::command(int narg, char **arg)
 
       // all remaining args are commands
       // first,last = arg index of first/last commands
-      // set ncommands = 0 if single command and it is nullptr
+      // set ncommands = 0 if single command and it is "NULL"
 
     } else if (strcmp(arg[iarg],"every") == 0) {
       if (iarg+3 > narg) error->all(FLERR,"Illegal run command");