So what I'm thinking now is that if PAGER is empty or all white space then we should not try to use it as a shell command; we can either treat the case as "pager off" or as "use default pager". Everything else we can leave to the invoked shell to complain about.
My pick would be to leave \pset pager to control the on/off nature of the pager and treat both an unset and an empty string PAGER identically and solely for the purpose of choosing which pager to use should one be required.