When --no-extensions was used, extensionsResult.extensions was an empty
array. The condition in buildSessionOptions() checked .length > 0,
so preloadedExtensions was not set. This caused createAgentSession()
to fall through to extension discovery.
Remove the .length > 0 condition so the empty result is passed through,
signaling that extension loading was already handled.
Fixes#776
3326b8f521