libpq:  have PQconnectdbParams() and PQpingParams accept "" as default

Previously, these functions treated "" optin values as defaults in some
ways, but not in others, like when comparing to .pgpass.  Also, add
documentation to clarify that now "" and NULL use defaults, like
PQsetdbLogin() has always done.

BACKWARD INCOMPATIBILITY

Patch by Adrian Vondendriesch, docs by me

Report by Jeff Janes