fix(js-sdk): use undefined check when forwarding snapshot name

Truthy check silently drops name: '' — check !== undefined instead so
the API can validate/reject an explicit empty string rather than the
SDK quietly omitting it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: FisherXZ

packages/js-sdk/src/sandbox/sandboxApi.ts

@@ -722,7 +722,7 @@ export class SandboxApi {
         },
       },
       body: {
-        ...(opts?.name && { name: opts.name }),
+        ...(opts?.name !== undefined && { name: opts.name }),
       },
       signal: config.getSignal(opts?.requestTimeoutMs),
     })