fix: prevent underflow when subgraphs go under the minimum signal threshold

[pcarranzav]

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Depends on #569

[codecov[bot]]

Codecov Report

Merging #640 (0ec8778) into dev (7c6e27d) will increase coverage by 0.22%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #640      +/-   ##
==========================================
+ Coverage   90.35%   90.57%   +0.22%     
==========================================
  Files          35       35              
  Lines        1762     1762              
  Branches      296      296              
==========================================
+ Hits         1592     1596       +4     
+ Misses        170      166       -4     

Flag	Coverage Δ
unittests	90.57% <100.00%> (+0.22%)	:arrow_up:

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Impacted Files	Coverage Δ
contracts/rewards/RewardsManager.sol	100.00% <100.00%> (+4.04%)	:arrow_up:

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