prevent PackWriter from using Notify if nothing was written

[erizocosmico]

If PackWriter is not used, Close will call Notify even if the index is nil, which may cause a nil pointer dereference later down the line in the execution in ObjectStorage due to the insertion of nil indexes on the index.

An alternative fix if this is correct behavior would be to just check for nil indexes here:

go-git/storage/filesystem/object.go

Lines 90 to 92 in 91868d1

	w.Notify = func(h plumbing.Hash, idx *packfile.Index) {
	s.index[h] = idx
	}

[codecov]

Codecov Report

Merging #543 into master will decrease coverage by 0.6%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #543      +/-   ##
==========================================
- Coverage   78.07%   77.47%   -0.61%     
==========================================
  Files         129      129              
  Lines        9792     9792              
==========================================
- Hits         7645     7586      -59     
- Misses       1316     1388      +72     
+ Partials      831      818      -13

Impacted Files	Coverage Δ
storage/filesystem/internal/dotgit/writers.go	77.03% <0%> (ø)	⬆️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️

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