Add Empty: an empty container proxy

[tkf]

No description provided.

[codecov-io]

Codecov Report

Merging #33 into master will increase coverage by 0.55%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   89.02%   89.58%   +0.55%     
==========================================
  Files          16       17       +1     
  Lines         237      240       +3     
==========================================
+ Hits          211      215       +4     
+ Misses         26       25       -1

Impacted Files	Coverage Δ
src/BangBang.jl	100% <ø> (ø)	⬆️
src/NoBang/NoBang.jl	100% <ø> (ø)	⬆️
src/NoBang/emptycontainers.jl	100% <100%> (ø)
src/NoBang/base.jl	95.52% <0%> (+1.49%)	⬆️

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