--after option for delete retain is throwing a panic error

[InteNs]

Database name

PostgreSQL

Issue description

Trying to delete backups according to our retention policy of 30 days.
We've been doing this with retain FULL 3 as we make full backups every 2 weeks, this is however never really 30 days ago.
Now i've been trying to get the --after option to work with retain FULL 1 so we always have a full backup, and everything from the last 30 days.

Describe your problem

the --after argument is not working at all, it gives the error as you can see in the logs below

Please provide steps to reproduce

wal-g delete retain FULL 1 --after 2024-03-20T00:00:00

Please add config and wal-g stdout/stderr logs for debug purpose

command log

```bash

wal-g delete retain 5 --after 2024-03-20T00:00:00

INFO: 2024/04/22 13:27:14.232305 Backup to delete will be searched in storages: [default]
INFO: 2024/04/22 13:27:14.232423 retrieving permanent objects
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x18fe24c]

goroutine 1 [running]:
github.com/wal-g/wal-g/internal/databases/postgres.timelineAndSegmentNoLess({0x7fd8846e05c0?, 0xc0001b4700?}, {0x0, 0x0})
/root/go/src/github.com/wal-g/wal-g/internal/databases/postgres/delete.go:204 +0x4c
github.com/wal-g/wal-g/internal.NewDeleteHandler.func1({0x0?, 0x0?}, {0x7fd8846e05c0?, 0xc0001b4700?})
/root/go/src/github.com/wal-g/wal-g/internal/delete_handler.go:91 +0x43
github.com/wal-g/wal-g/internal.(*DeleteHandler).FindTargetRetainAfterName(0xc000d8f840, 0x19?, {0x7fff8c61fb71, 0x13}, 0x0)
/root/go/src/github.com/wal-g/wal-g/internal/delete_handler.go:303 +0x23c
github.com/wal-g/wal-g/internal.(*DeleteHandler).FindTargetRetainAfter(0x7fff8c61fb67?, 0x1?, {0x7fff8c61fb71, 0x13}, 0x6c?)
/root/go/src/github.com/wal-g/wal-g/internal/delete_handler.go:275 +0x76
github.com/wal-g/wal-g/internal.(*DeleteHandler).HandleDeleteRetainAfter(0x25abc40?, {0xc000b37920?, 0xac?, 0xab?}, 0x0?)
/root/go/src/github.com/wal-g/wal-g/internal/delete_handler.go:148 +0x229
github.com/wal-g/wal-g/cmd/pg.runDeleteRetain(0x3b83ee0?, {0xc000b37920?, 0x1, 0x3})
/root/go/src/github.com/wal-g/wal-g/cmd/pg/delete.go:91 +0x1f2
github.com/spf13/cobra.(*Command).execute(0x3b83ee0, {0xc000b378f0, 0x3, 0x3})
/root/go/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:860 +0x663
github.com/spf13/cobra.(*Command).ExecuteC(0x3b80f60)
/root/go/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:974 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
/root/go/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:902
github.com/wal-g/wal-g/cmd/pg.Execute()
/root/go/src/github.com/wal-g/wal-g/cmd/pg/pg.go:53 +0x25
main.main()
/root/go/src/github.com/wal-g/wal-g/main/pg/main.go:8 +0x17
root@a67b4670221f:/# root@a67b4670221f:/# wal-g delete retain 5 --after 2024-03-20T00:00:00
INFO: 2024/04/22 13:27:14.232305 Backup to delete will be searched in storages: [default]
INFO: 2024/04/22 13:27:14.232423 retrieving permanent objects
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x18fe24c]

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