handle isolated nodes

[CarloLucibello]

Fix #138

[codecov]

Codecov Report

Merging #139 (a6a9c49) into master (77f0e0f) will increase coverage by 0.02%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   85.36%   85.38%   +0.02%     
==========================================
  Files          15       15              
  Lines        1257     1259       +2     
==========================================
+ Hits         1073     1075       +2     
  Misses        184      184              

Impacted Files	Coverage Δ
src/GNNGraphs/gatherscatter.jl	80.00% <83.33%> (+5.00%)	⬆️
src/GNNGraphs/transform.jl	96.74% <100.00%> (ø)
src/msgpass.jl	64.28% <100.00%> (ø)

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