cli: skip migrating global flags for nil action (#548)

> ./build/bin/ronin account

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x10035aa]

goroutine 1 [running]:
github.com/ethereum/go-ethereum/internal/flags.MigrateGlobalFlags.func2.1(0xc0004eeec0)
        github.com/ethereum/go-ethereum/internal/flags/helpers.go:83 +0x2a
github.com/urfave/cli/v2.(*Command).Run(0x29b36c0, 0xc0004eeec0, {0xc0006da160, 0x1, 0x1})
        github.com/urfave/cli/[email protected]/command.go:279 +0x97d
github.com/urfave/cli/v2.(*Command).Run(0xc0005eec60, 0xc0004cc080, {0xc000156000, 0x2, 0x2})
        github.com/urfave/cli/[email protected]/command.go:272 +0xbb7
github.com/urfave/cli/v2.(*App).RunContext(0xc00028da00, {0x1d92a58, 0x2c71480}, {0xc000156000, 0x2, 0x2})
        github.com/urfave/cli/[email protected]/app.go:337 +0x58b
github.com/urfave/cli/v2.(*App).Run(...)
        github.com/urfave/cli/[email protected]/app.go:311
main.main()
        github.com/ethereum/go-ethereum/cmd/ronin/main.go:308 +0x45

Currently, we get the crash because the account command does not define any
actions, it requires the user to provide further subcommand. Normally, when
having a command with nil action, the cli package will print the helper.
However, since we want the global flags to be available to subcommand, we
create a wrapper action which migrates the global flags before calling the
underlying action. And we create a wrapper action without checking the
underlying action is nil which leads to nil action to be called. This commit
skips creating the wrapper action to migrate flags when the underlying action is
nil.

internal/flags/helpers.go

@@ -78,9 +78,11 @@ func MigrateGlobalFlags(ctx *cli.Context) {
 	// This iterates over all commands and wraps their action function.
 	iterate(ctx.App.Commands, func(cmd *cli.Command) {
 		action := cmd.Action
-		cmd.Action = func(ctx *cli.Context) error {
-			doMigrateFlags(ctx)
-			return action(ctx)
+		if action != nil {
+			cmd.Action = func(ctx *cli.Context) error {
+				doMigrateFlags(ctx)
+				return action(ctx)
+			}
 		}
 	})
 }