pgsql: Silence Perl warning

Silence Perl warning

Now that warnings are enabled across the board, this code that tries to
print an undef variable emits one.  Silently printing the empty string
achieves the previous behavior.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
Discussion: https://postgr.es/m/E1jO1VT-0008Qk-TM@gemulon.postgresql.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/e56d717d8a23c48e4ab27b6bd6815c50dc3a378a

Modified Files
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doc/src/sgml/mk_feature_tables.pl | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)