[MC] Eagerly skip zero-sized .fill fragments

This doesn't change the output in any way, but we have a bunch of
emitFill for padding. When emitting an array of floats we'd end up with

DataFragment float1
FillFragment 0
DataFragment float2
FillFragment 0
... and so on

We never actually emit anything for those fills, neither in asm nor obj
emission mode, they just consume RAM for no reason.