catchup: Fix empty cert if ledger already has a block

[algorandskiy]

Summary

This fixes nil pointer dereference in StoreBlock/StoreFirstBlock discovered by e2e catchup test.

algod(22258) : github.com/algorand/go-algorand/ledger.(*catchpointCatchupAccessorImpl).StoreBlock(0xc00836e480, {0x2ee95c8652e0cbe8?, 0xad49081e76d4ff29?}, 0xc009b79880, 0x0)
algod(22258) : 	github.com/algorand/go-algorand/ledger/catchupaccessor.go:1078 +0x1a0

It was introduced in #5798 and very similar to the psp fix in #4390

Test Plan

• e2e tests pass
• add a unit test

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (3abe5c8) 55.83% compared to head (14d4f08) 55.87%.

Files	Patch %	Lines
catchup/catchpointService.go	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5846      +/-   ##
==========================================
+ Coverage   55.83%   55.87%   +0.03%     
==========================================
  Files         477      477              
  Lines       67256    67259       +3     
==========================================
+ Hits        37552    37580      +28     
+ Misses      27171    27131      -40     
- Partials     2533     2548      +15     

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