pgsql: Fix compilation without injection points with some compilers

Fix compilation without injection points with some compilers

Some compilers didn't like the empty initializer when compiled without
USE_INJECTION_POINTS. Per buildfarm member 'drongo', using Visual
Studio 2019.

Author: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/adNHcBVJO5gIOp1l@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2670a0fcc6a5ee0273b0751dcc18d514735290ad

Modified Files
--------------
src/backend/utils/misc/injection_point.c | 6 +-----
src/include/storage/subsystemlist.h      | 2 ++
2 files changed, 3 insertions(+), 5 deletions(-)