Work around annoying warning on GCC 10.2.
Work around annoying warning on GCC 10.2.

GCC warns if N == 0 in SmallVector and simpler workarounds do nothing.

KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross