It is certainly too small to have any practical significance for the definition of the metre in the foreseeable future but it cannot be shown to be exactly zero even though currently accepted theories indicate that it is. If it wasn't zero, the speed of light would not be constant but from a theoretical point of view we would then take c to be the upper limit of the speed of light in vacuum so that we can continue to ask if c is constant. Previously the metre and second have been defined in various different ways according to the measurement techniques of the time. They could change again in the future. We now know that there are variations in the length of a mean solar day as measured by atomic clocks.
Standard time is adjusted by adding or subtracting a leap second from time to time.
There may have been even larger variations in the length or the metre standard caused by metal shrinkage. Obviously it would be more natural to attribute those changes to variations in the units of measurement than to changes in the speed of light itself, but by the same token it is nonsense to say that the speed of light is now constant just because the SI definitions of units define its numerical value to be constant. But the SI definition highlights the point that we need to first be very clear what we mean by constancy of the speed of light before we answer our question. We have to state what we are going to use as our standard ruler and our standard clock when we measure c . We could, for example, take the definitions of the units as they stood between 1967 and 1983.