Make `Source` field in `Breakpoint` events optional

[corneliusweig]

Currently, an empty breakpoint contains a "source" field with an empty object:

data, _ := json.Marshal(dap.Breakpoint{})
fmt.Println(string(data))

// {"verified":false,"source":{}}
//                     ^^^^^^^^

This means that BreakpointEvent types with missing source are serialised with an empty source object, which clears the breakpoint location in the UI.

This change makes the "source" field optional, so that an empty breakpoint serialises as:

data, _ := json.Marshal(dap.Breakpoint{})
fmt.Println(string(data))

// {"verified":false}

Fixes #66

[suzmue]

Thanks for reporting and fixing this bug!

cmd/gentypes/gentypes.go is used to generate schematypes.go so that will need to be updated as well.

[suzmue]

Hi @corneliusweig! Just marked this as ready for review so that we can get this change in.

[corneliusweig]

Thank you @suzmue for adding the missing bits!