Fix: Walk shouldn't return empty baseName (i.e. object representing the directory itself)

[gldeng]

Fix to the following issue:

Issue: Empty baseName causes Merger, Relayer and Firehose services to fail.
Root Cause: When walking GCP bucket directory, it returns the directory as the first object

panic: wrong filename format: ""

goroutine 101 [running]:
github.com/streamingfast/bstream.MustNewOneBlockFile(...)
        /home/runner/go/pkg/mod/github.com/streamingfast/[email protected]/oneblockfile.go:107
github.com/streamingfast/firehose-core/merger.(*ForkAwareDStoreIO).DeleteForkedBlocksAsync.func1({0xc00114b5f0?, 0xc00114b5f0?})
        /home/runner/go/pkg/mod/github.com/streamingfast/[email protected]/merger/merger_io.go:317 +0x13c
github.com/streamingfast/dstore.(*GSStore).WalkFrom(0xc000702ba0, {0x3144838, 0x4aef260}, {0x0, 0x0}, {0x0, 0x0}, 0xc0023fefa8)
        /home/runner/go/pkg/mod/github.com/streamingfast/[email protected]/gsstore.go:291 +0x727
github.com/streamingfast/firehose-core/merger.(*ForkAwareDStoreIO).DeleteForkedBlocksAsync(0xc00093f500, 0x1, 0x0)
        /home/runner/go/pkg/mod/github.com/streamingfast/[email protected]/merger/merger_io.go:313 +0xdb
github.com/streamingfast/firehose-core/merger.(*Merger).startForkedBlocksPruner.func1()
        /home/runner/go/pkg/mod/github.com/streamingfast/[email protected]/merger/merger.go:104 +0xa2
created by github.com/streamingfast/firehose-core/merger.(*Merger).startForkedBlocksPruner in goroutine 97
        /home/runner/go/pkg/mod/github.com/streamingfast/[email protected]/merger/merger.go:97 +0x225

[maoueh]

Is this happening because you are using a GCP bucket directly without any "directory"?