Support isEmptyStringInfo

Hi All,
We have (StringInfo::len == 0) checks at many places. I thought it
would be better to wrap that into a function isEmptyStringInfo() to
make those checks more readable and also abstract the logic to check
emptiness of a StringInfo. I think this will be useful to extensions
outside core which also have these checks. They won't need to worry
about that logic/code being changed in future; rare but not impossible
case.

Probably we should have similar support for PQExpBuffer as well, which
will be more useful to hide the internals of PQExpBuffer from client
code. But I haven't included those changes in this patch. I can do
that if hackers like the idea.

--
Best Wishes,
Ashutosh Bapat