fix: prevent unpausing gateways if they're not configured (OZ M-01 from #700)

[pcarranzav]

Based on a finding from OZ's audit of #700, but targeting #699 as the change is relevant to that branch too.

[codecov]

Codecov Report

Base: 91.53% // Head: 91.57% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (6d4a0d7) compared to base (323bdf8).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@                Coverage Diff                @@
##           pcv/l2-bridge     #733      +/-   ##
=================================================
+ Coverage          91.53%   91.57%   +0.04%     
=================================================
  Files                 42       42              
  Lines               1984     1995      +11     
  Branches             346      354       +8     
=================================================
+ Hits                1816     1827      +11     
  Misses               168      168              

Flag	Coverage Δ
unittests	91.57% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
contracts/gateway/GraphTokenGateway.sol	100.00% <100.00%> (ø)
contracts/gateway/L1GraphTokenGateway.sol	100.00% <100.00%> (ø)
contracts/l2/gateway/L2GraphTokenGateway.sol	100.00% <100.00%> (ø)

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[tmigone]

Same change as https://github.com/graphprotocol/contracts/pull/717/files#diff-729e91d21c664fed317c8bccc772f1c5cce6a8a4d943d700359ad371302aef55

which is pending merge after audit. I'd still make the change here and later we solve the conflict whenever it shows up.

[pcarranzav]

Oooh oops, sorry I missed that. But yeah, let's keep the change here so that we have e2e passing here.