Fix empty combinations of observables

[bvdmitri]

This PR fixes empty combinations of observables. For convenience empty combinations of observables send completion event immediately.

[codecov-commenter]

Codecov Report

Merging #23 (75ecb8d) into master (d56e553) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   77.79%   77.83%   +0.03%     
==========================================
  Files         115      115              
  Lines        3517     3523       +6     
==========================================
+ Hits         2736     2742       +6     
  Misses        781      781              

Impacted Files	Coverage Δ
src/observable/collected.jl	95.45% <100.00%> (+0.14%)	⬆️
src/observable/combined.jl	91.52% <100.00%> (+0.29%)	⬆️
src/observable/combined_updates.jl	92.85% <100.00%> (+0.26%)	⬆️

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