Refuse to create empty snapshots

[fd0]

Closes #862

[codecov-io]

Codecov Report

Merging #865 into master will increase coverage by 0.13%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #865      +/-   ##
==========================================
+ Coverage   53.44%   53.58%   +0.13%     
==========================================
  Files          92       92              
  Lines        7624     7629       +5     
==========================================
+ Hits         4075     4088      +13     
+ Misses       2934     2927       -7     
+ Partials      615      614       -1

Impacted Files	Coverage Δ
src/restic/archiver/archiver.go	64.96% <75%> (+2.18%)	✅

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