[lldb-vscode] Allow an empty 'breakpoints' field to clear breakpoints.

Per the DAP spec for SetBreakpoints [1], the way to clear breakpoints is: `To clear all breakpoint for a source, specify an empty array.`

However, leaving the breakpoints field unset is also a well formed request (note the `breakpoints?:` in the `SetBreakpointsArguments` definition). If it's unset, we have a couple choices:

1. Crash (current behavior)
2. Clear breakpoints
3. Return an error response that the breakpoints field is missing.

I propose we do (2) instead of (1), and treat an unset breakpoints field the same as an empty breakpoints field.

[1] https://microsoft.github.io/debug-adapter-protocol/specification#Requests_SetBreakpoints

Reviewed By: wallace, labath

Differential Revision: https://reviews.llvm.org/D88513