pset, NULL, and COPY-reporting

Greetings --

Due to some level of ignorance at install-time, our "psql" clients on two
different production computers appear to have different values for /null/: ""
and "NULL" respectively.
At the same time, one of these computers reports the number of rows read-in with
a "\copy TABLE FROM FILE" command as "COPY 123", while the other computer does
not.

Is there some reference that tells specifically where the "default at login"
value of /null/ is set?
Is there some reference that tells what controls the "COPY 123" behavior?
Are the /null/ and COPY behaviors linked? If so, how?

Thank you for your consideration,
John Bell
ARM Archive
Oak Ridge National Lab