video/out/filter_kernels: fall back to nearest tap when sum is zero

When the effective filter bound (radius * blur) is smaller than the
smallest tap distance, mp_compute_weights produces all-zero weights and
the subsequent normalization divides by zero, producing NaN weights that
render as black. This can happen for resizable filters where the user
picks a radius below 0.5.

Detect sum == 0 and fall back to nearest-neighbor on the closest tap.