bug(cli): `mcap cat` Sigsev on schemaless encoding

[jon-chuang]

Description
The following error is encountered:

running /tmp/tmpbiz8i177
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xb77ca5]

goroutine 1 [running]:
github.com/foxglove/mcap/go/cli/mcap/cmd.printMessages({0xc00034fc40?, 0xc000355480?}, {0xf5fee0, 0xc000010018}, {0xf5cd80, 0xc00050c1c0}, 0x1)
	/home/jonch/Desktop/Programming/systems/mcap/go/cli/mcap/cmd/cat.go:113 +0x4e5
github.com/foxglove/mcap/go/cli/mcap/cmd.glob..func5.1(0x75?, {0x7fe3f42e82f8?, 0xc000010af8})
	/home/jonch/Desktop/Programming/systems/mcap/go/cli/mcap/cmd/cat.go:236 +0x1c5
github.com/foxglove/mcap/go/cli/mcap/utils.WithReader({0xf64f50, 0xc0000360d8}, {0x7ffe00805e75?, 0x0?}, 0xc0002dfd90)
	/home/jonch/Desktop/Programming/systems/mcap/go/cli/mcap/utils/utils.go:104 +0x431
github.com/foxglove/mcap/go/cli/mcap/cmd.glob..func5(0x145c420?, {0xc00009b980, 0x1, 0x2?})
	/home/jonch/Desktop/Programming/systems/mcap/go/cli/mcap/cmd/cat.go:226 +0x312
github.com/spf13/cobra.(*Command).execute(0x145c420, {0xc00009b960, 0x2, 0x2})
	/home/jonch/go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:876 +0x67b
github.com/spf13/cobra.(*Command).ExecuteC(0x145b520)
	/home/jonch/go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:990 +0x3b4
github.com/spf13/cobra.(*Command).Execute(...)
	/home/jonch/go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:918
github.com/foxglove/mcap/go/cli/mcap/cmd.Execute()
	/home/jonch/Desktop/Programming/systems/mcap/go/cli/mcap/cmd/root.go:21 +0x25
main.main()
	/home/jonch/Desktop/Programming/systems/mcap/go/cli/mcap/main.go:6 +0x17
CompletedProcess(args=['mcap', 'cat', '--json', '/tmp/tmpbiz8i177'], returncode=2, stdout=b'')

• Version: 1.0.2
• Platform: Ubuntu 22.04

Steps To Reproduce
In python/mcap/tests/test_writer.py:

def test_generate_sample_data():
    file = NamedTemporaryFile("w+b")
    writer = Writer(file, compression=CompressionType.ZSTD)
    writer.start(library="test")

    channel_id = writer.register_channel(
        schema_id=0,
        topic="sample_topic",
        message_encoding="json",
    )

    writer.add_message(
        channel_id=channel_id,
        log_time=0,
        data=json.dumps({"sample": "test"}).encode(encoding="utf-8"),
        publish_time=0,
    )

    writer.finish()
    file.seek(0)
    print("running", file.name)
    result = subprocess.run(['mcap', 'cat', '--json', file.name], stdout=subprocess.PIPE)
    print(result)

if __name__ == "__main__":
    test_generate_sample_data()

Expected Behavior
No Sigsev. Note that running with a schema works.

See the comment for mcap.Writer.register_channel:

A schema_id of 0 indicates there is no schema for this channel.

[foxhubber]

Linear: FG-2431

[jon-chuang]

Seems that the schema is a null pointer.

We should handle it as such.