Does not work without specifying schema

When using the binary from the release page or building master from the source, the command fails when a schema has not been specified:

[!] No schema specified, will run without table and column validation.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x870c29]

goroutine 1 [running]:
main.(*SQLVet).Vet(0xc000150ee0)
	/home/runner/work/sqlvet/sqlvet/main.go:43 +0x29
main.main.func2(0xc000189700?, {0xc00011ebd0?, 0x7?, 0xb7435e?})
	/home/runner/work/sqlvet/sqlvet/main.go:147 +0x45
github.com/spf13/cobra.(*Command).execute(0xc00016d808, {0xc000132010, 0x1, 0x1})
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:920 +0x867
github.com/spf13/cobra.(*Command).ExecuteC(0xc00016d808)
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:1044 +0x3a5
github.com/spf13/cobra.(*Command).Execute(...)
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:968
main.main()
	/home/runner/work/sqlvet/sqlvet/main.go:166 +0x1d6

It appears that s.Schema is nil in vet.NewContext(tables: s.Schema.Tables).

Start in main.go at SQLVet.Vet and the vet.NewContext(tables: s.Schema.Tables) call identified in the report. Reproduce the command with no schema using the release binary or a build from master. Done means it no longer panics and can run without table and column validation while preserving the reported warning.