fix command when not passing arguments to capture

[alegrey91]

The program panics when running harpoon capture -f main.doSomething without specifying a command to run.
Eg.

panic: runtime error: index out of range [0] with length 0                                                                                                                                                                                    
                                                                                                                                                                                                                                              
goroutine 1 [running]:                                                                                                                                                                                                                        
github.com/alegrey91/harpoon/cmd.glob..func1(0xc000148100?, {0xc00007a140, 0x0, 0x5})                                                                                                                                                         
        /home/runner/work/harpoon/harpoon/cmd/capture.go:113 +0x13f5                                                                                                                                                                          
github.com/spf13/cobra.(*Command).execute(0x874020, {0xc00007a0f0, 0x5, 0x5})                                                                                                                                                                 
        /home/runner/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:987 +0xaa3                                                                                                                                                           
github.com/spf13/cobra.(*Command).ExecuteC(0x874300)                                                                                                                                                                                          
        /home/runner/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1115 +0x3ff                                                                                                                                                          
github.com/spf13/cobra.(*Command).Execute(...)                                                                                                                                                                                                
        /home/runner/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1039                                                                                                                                                                 
github.com/alegrey91/harpoon/cmd.Execute()                                                                                                                                                                                                    
        /home/runner/work/harpoon/harpoon/cmd/root.go:37 +0x1a                                                                                                                                                                                
main.main()                                                                                                            
        /home/runner/work/harpoon/harpoon/main.go:21 +0xf

This is because the code is not checking if we have enough arguments in input to run.