fix: pass correct readers for legacy sealed fraction

[dkharms]

Description

When trying to compact legacy fractions following panic occurs:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0xbb4a17]

goroutine 173 [running]:
github.com/ozontech/seq-db/storage.(*ReaderProvider).GetReader(0x0)
    github.com/ozontech/seq-db/storage/index_reader.go:37 +0x57
github.com/ozontech/seq-db/frac.(*Sealed).mustGetReader(0xc0002a68c0, 0x1602a01?)
    github.com/ozontech/seq-db/frac/sealed.go:283 +0x51
github.com/ozontech/seq-db/frac.NewSealedSource(0xc0002a68c0)
    github.com/ozontech/seq-db/frac/sealed_source.go:34 +0x57
github.com/ozontech/seq-db/compaction.(*Executor).compact(0xc0004a6280, {{0x0, 0xee1e04ee0, 0x2395120}, {0xc000115038, 0x6}, {0xc000244090, 0x27}, 0xc0006e83c0, 0xc0004c0600})
    github.com/ozontech/seq-db/compaction/executor.go:60 +0x2ea
github.com/ozontech/seq-db/compaction.NewExecutor.(*Executor).init.func1()
    github.com/ozontech/seq-db/compaction/executor.go:41 +0x145
sync.(*WaitGroup).Go.func1()
    sync/waitgroup.go:239 +0x4a
created by sync.(*WaitGroup).Go in goroutine 1
    sync/waitgroup.go:237 +0x73

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.14286% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.55%. Comparing base (94e5bd0) to head (b1d6051).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
frac/sealed_source.go	57.14%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #508      +/-   ##
==========================================
- Coverage   71.56%   71.55%   -0.02%     
==========================================
  Files         239      239              
  Lines       20158    20161       +3     
==========================================
  Hits        14426    14426              
- Misses       4652     4653       +1     
- Partials     1080     1082       +2     

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[eguguchkin]

I'll approve this, but I think it would be better to merge this same fix as part of another PR

[dkharms]

Will be fixed in #489