refactor: simplify zero implementation

[tri-adam]

Replace func zeroData with a simpler implementation that uses an io.Reader that streams zero bytes.

[codecov]

Codecov Report

Merging #328 (59cf700) into main (e1f38f0) will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
+ Coverage   72.77%   72.80%   +0.02%     
==========================================
  Files          35       35              
  Lines        2964     2956       -8     
==========================================
- Hits         2157     2152       -5     
+ Misses        648      646       -2     
+ Partials      159      158       -1     

Files Changed	Coverage Δ
pkg/sif/create.go	71.13% <66.66%> (+0.17%)	⬆️