mcap merge fails on missing schema

[vivek-rane]

Description
When merging 2 mcaps with the CLI tool, and one of them has a schema ID of 0 (no schema, just 1 channel with schema ID 0 and 1 message on that channel), the CLI tool crashes with:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1006df838]

goroutine 1 [running]:
github.com/foxglove/mcap/go/cli/mcap/cmd.(*mcapMerger).addSchema(0x14003405d48, 0x0?, 0x1c1, 0x0)
        /private/tmp/mcap-20230216-70145-8qbk2v/mcap-releases-mcap-cli-v0.0.25/go/cli/mcap/cmd/merge.go:109 +0x28
github.com/foxglove/mcap/go/cli/mcap/cmd.(*mcapMerger).mergeInputs(0x14003405d48, {0x100acecc0, 0x14000011908}, {0x140004d0000, 0x1c4, 0x0?})
        /private/tmp/mcap-20230216-70145-8qbk2v/mcap-releases-mcap-cli-v0.0.25/go/cli/mcap/cmd/merge.go:186 +0x6e4
github.com/foxglove/mcap/go/cli/mcap/cmd.glob..func12(0x100ff6060?, {0x140004ae000?, 0x1c4, 0x200?})
        /private/tmp/mcap-20230216-70145-8qbk2v/mcap-releases-mcap-cli-v0.0.25/go/cli/mcap/cmd/merge.go:279 +0x29c
github.com/spf13/cobra.(*Command).execute(0x100ff6060, {0x140004a6000, 0x1c6, 0x200})
        /Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:876 +0x4b8
github.com/spf13/cobra.(*Command).ExecuteC(0x100ff5de0)
        /Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:990 +0x354
github.com/spf13/cobra.(*Command).Execute(...)
        /Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:918
github.com/foxglove/mcap/go/cli/mcap/cmd.Execute()
        /private/tmp/mcap-20230216-70145-8qbk2v/mcap-releases-mcap-cli-v0.0.25/go/cli/mcap/cmd/root.go:21 +0x28
main.main()
        /private/tmp/mcap-20230216-70145-8qbk2v/mcap-releases-mcap-cli-v0.0.25/go/cli/mcap/main.go:6 +0x1c

• Version: mcap version v0.0.25
• Platform: macOS Monterey 12.5.1

Steps To Reproduce

• Create an mcap with the python API with 1 channel set to schema ID 0 and 1 or more messages
• Run the merge command with mcap merge no-schema.mcap regular.mcap
• Tool crashes with runtime error above

Expected Behavior
Tool should generate merged mcap so that it can be loaded into Foxglove studio (ref: #5351 Support schemaless channels)

[foxhubber]

Linear: FG-2122