Put nano generated code into nano package, unless option javanano_use_deprecated_package is set.
Also fix the behaior while java_package is set to empty.
diff --git a/compiler/src/test/golden/TestServiceNano.java.txt b/compiler/src/test/golden/TestServiceNano.java.txt
index 4012e40..0314a8d 100644
--- a/compiler/src/test/golden/TestServiceNano.java.txt
+++ b/compiler/src/test/golden/TestServiceNano.java.txt
@@ -1,4 +1,4 @@
-package io.grpc.testing.integration;
+package io.grpc.testing.integration.nano;
import static io.grpc.stub.ClientCalls.asyncUnaryCall;
import static io.grpc.stub.ClientCalls.asyncServerStreamingCall;