fix: remove unneeded empty bytes from callhook data

[pcarranzav]

With the way it worked up to now, _data.length would always be nonzero, so we were calling the callhooks always, even when transferring to an EOA! This removes those unnecessary emptyBytes (which were there just to keep the gateway similar to Arbitrum's gateway implementation), and now _data on the L2 side will be the callhook data sent from L1, and therefore zero length if there is no callhook to call.

[codecov]

Codecov Report

Base: 91.60% // Head: 91.58% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (caf5ab5) compared to base (ce0d0a3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@                Coverage Diff                @@
##           pcv/c4-699-qa     #745      +/-   ##
=================================================
- Coverage          91.60%   91.58%   -0.02%     
=================================================
  Files                 42       42              
  Lines               2000     1997       -3     
  Branches             361      361              
=================================================
- Hits                1832     1829       -3     
  Misses               168      168              

Flag	Coverage Δ
unittests	91.58% <100.00%> (-0.02%)	⬇️

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

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

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