code cleanup empty string initializations

In initdb, many global string variables are initialized as empty strings
("") and then checked later with strcmp(), instead of just using NULL.
I think this is probably left over from the shell script conversion.
The style has also spread to pg_basebackup. So here is a patch to clean
that up, and a second patch to clean up some other a bit confusing
business in initdb that seems like old shell script code.

While looking around, I found some useless empty string initializations
in the ecpg test suite as well, and here is another patch for that.

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Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services