[mono][debugger] Don't pass uninitialized context to debugger (#86625)

[mono][debugger] Don't pass uninitialized context to debugger (#86625)

mono_dbg_debugger_agent_user_break first tries to initialize a context before sending it to the debugger machinery. If, during unwinding, we don't have a saved context, we were passing uninitialized data (with interpreter for example). On some architectures this was making unwinding break in mono_walk_stack_full, because the stack pointer was invalid. This change makes it so we at least pass a valid context.