feat(share): Ensure empty CAR exists in eds.Store

[distractedm1nd]

Overview

Closes #1113
This PR adds an empty CAR to the eds.Store during startup of the store.

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[codecov-commenter]

Codecov Report

Merging #1409 (d1afb9c) into main (f518fc4) will decrease coverage by 0.07%.
The diff coverage is 60.60%.

@@            Coverage Diff             @@
##             main    #1409      +/-   ##
==========================================
- Coverage   55.57%   55.50%   -0.08%     
==========================================
  Files         203      203              
  Lines       12144    12151       +7     
==========================================
- Hits         6749     6744       -5     
- Misses       4726     4734       +8     
- Partials      669      673       +4     

Impacted Files	Coverage Δ
share/eds/store.go	54.21% <ø> (+2.10%)	⬆️
nodebuilder/share/module.go	72.54% <11.11%> (-4.54%)	⬇️
nodebuilder/share/constructors.go	92.10% <66.66%> (-7.90%)	⬇️
share/empty.go	86.66% <86.66%> (-13.34%)	⬇️
share/eds/byzantine/bad_encoding.go	66.00% <0.00%> (-3.00%)	⬇️
share/eds/byzantine/pb/share.pb.go	32.88% <0.00%> (-1.97%)	⬇️
nodebuilder/node/admin.go	25.00% <0.00%> (-1.67%)	⬇️
share/ipld/get.go	92.18% <0.00%> (-1.24%)	⬇️
nodebuilder/p2p/host.go	90.62% <0.00%> (-0.29%)	⬇️
share/ipld/proof.go	100.00% <0.00%> (ø)
... and 14 more

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[Wondertan]

See the comment.

We also should not merge this before we do v0.5.0