nir: Don't use empty initializer list in lower_io_arrays

Using empty initializer list in C code causes compilation
issues with MSVC. gcc and clang display warnings (if the
warning level is high enough), but do not fail the build.
MSVC does.

Fixes: ee2050b1
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
49 jobs for !2472 with fix-msvc-compilation in 29 minutes and 32 seconds (queued for 7 seconds)
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