Handle unavailable sources in compliance with DAP spec

[playdohface]

Fixes #605

• when the source is not available, e.g. for native methods or lambdas, return 0 in the line field of StackFrame as prescribed by the Debug Adapter Protocol Specification
• set the display value of the source path to "unknown" explicitly instead of returning null source. This is not required by the spec, but friendly for clients

[chagong]

It's good to go. Thanks for the contribution. @playdohface

[mfussenegger]

This is breaking nvim-dap and from what I can tell not compliant with the DAP spec:

      }, {
        column = 1,
        id = 148,
        line = 0,
        name = "0x00000000a114abb0.run()",
        presentationHint = "subtle",
        source = {
          name = "Unknown Source",
          path = "unknown",
          sourceReference = 0
        }
      }, {

According to https://microsoft.github.io/debug-adapter-protocol/specification#Types_Source path is nullable, which for a client is a more reasonable value than unknown which if interpreted as a path will raise unknown file errors, unless the user happens to have a /unknown file on the filesystem - in which case it would still be wrong.

But I think even clearer would be to keep source null - there isn't one after all.

(Also note that line=0 may not be enough for a client to identify if source is available or not if it uses linesStartAt1 = false in the initialize request)

[playdohface]

But I think even clearer would be to keep source null - there isn't one after all.

@mfussenegger Yes, I read the spec again and I agree that the spirit of the spec probably intends for an absent source in this case. The reason why I added an explicit "unknown" source was because that was required to get it working with Zed, but it turns out that probably both sides were to blame here: java-debug for setting line=-1 which violates the spec, and Zed for not handling the source=null case. For reference. zed-industries/zed#37080

I will work on a downstream fix for Zed, and would ask to merge #614