debugger: Mark DebugAdapterBinary::program as optional (#32534)

This allows us to support debugging with a debug adapter not managed by
Zed. Note that this is not a user facing change, as DebugAdapterBinary
is used to determine how to spawn a debugger. Thus, this should not
break any configs or anything like that.

Closes #ISSUE

Release Notes:

- N/A

crates/project/src/debugger/dap_store.rs

@@ -258,14 +258,17 @@ impl DapStore {
 
                     let (program, args) = wrap_for_ssh(
                         &ssh_command,
-                        Some((&binary.command, &binary.arguments)),
+                        binary
+                            .command
+                            .as_ref()
+                            .map(|command| (command, &binary.arguments)),
                         binary.cwd.as_deref(),
                         binary.envs,
                         None,
                     );
 
                     Ok(DebugAdapterBinary {
-                        command: program,
+                        command: Some(program),
                         arguments: args,
                         envs: HashMap::default(),
                         cwd: None,