fix: provide default message content to avoid nil pointer dereference

[MegaGrindStone]

In the current version, if user is executing pattern without input, eg fabric --pattern summarize_git_diff, the program will panic with this error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0xe22330]

goroutine 1 [running]:
github.com/danielmiessler/fabric/core.(*Chatter).BuildSession(0xc0004e43c0, 0xc00051e060, 0x0)
        /home/gs/go/pkg/mod/github.com/danielmiessler/[email protected]/core/chatter.go:116 +0x3b0
github.com/danielmiessler/fabric/core.(*Chatter).Send(0xc0004e43c0, 0xc00051e060, 0xc0003f00a0)
        /home/gs/go/pkg/mod/github.com/danielmiessler/[email protected]/core/chatter.go:30 +0x69
github.com/danielmiessler/fabric/cli.Cli({0x129b59d, 0x8})
        /home/gs/go/pkg/mod/github.com/danielmiessler/[email protected]/cli/cli.go:230 +0x7fc
main.main()
        /home/gs/go/pkg/mod/github.com/danielmiessler/[email protected]/main.go:12 +0x25

This PR address that issue by giving an empty input, or messageContent if request.Message is nil.