refactor: Re-add empty-string guard in setPoolLoadBalancingAlgorithm

With *string for LoadBalancingAlgorithm on this branch, makeEndpoint
produces "" when the NATS field is absent (keepalive registrations).
The guard prevents keepalives from triggering spurious algorithm
changes or error logs.

Author: hoffmaen

src/code.cloudfoundry.org/gorouter/route/pool.go

@@ -667,7 +667,12 @@ func (p *EndpointPool) MarshalJSON() ([]byte, error) {
 }
 
 // setPoolLoadBalancingAlgorithm overwrites the load balancing algorithm of a pool by that of a specified endpoint, if that is valid.
+// An empty algorithm (from a keepalive registration without algorithm field) is treated as "no change".
 func (p *EndpointPool) setPoolLoadBalancingAlgorithm(endpoint *Endpoint) {
+	if endpoint.LoadBalancingAlgorithm == "" {
+		return
+	}
+
 	if endpoint.LoadBalancingAlgorithm != p.LoadBalancingAlgorithm {
 		if config.IsLoadBalancingAlgorithmValid(endpoint.LoadBalancingAlgorithm) {
 			previousAlgorithm := p.LoadBalancingAlgorithm

src/code.cloudfoundry.org/gorouter/route/pool_test.go

@@ -505,6 +505,34 @@ var _ = Describe("EndpointPool", func() {
 				Expect(pool.HashLookupTable).To(BeNil())
 				Expect(pool.HashRoutingProperties).To(BeNil())
 			})
+
+			It("keeps the current algorithm when the endpoint does not specify one", func() {
+				pool := route.NewPool(&route.PoolOpts{
+					Logger:                 logger.Logger,
+					LoadBalancingAlgorithm: config.LOAD_BALANCE_RR,
+				})
+
+				// Set up HB routing
+				hbEndpoint := route.NewEndpoint(&route.EndpointOpts{
+					Host:                   "host-1",
+					Port:                   1234,
+					PrivateInstanceId:      "id-1",
+					LoadBalancingAlgorithm: config.LOAD_BALANCE_HB,
+					HashBalanceFactor:      1.25,
+					HashHeaderName:         "X-Tenant",
+				})
+				pool.Put(hbEndpoint)
+				Expect(pool.LoadBalancingAlgorithm).To(Equal(config.LOAD_BALANCE_HB))
+
+				// Register with empty algorithm (keepalive, field not specified) — should NOT reset
+				noAlgoEndpoint := route.NewEndpoint(&route.EndpointOpts{
+					Host:              "host-1",
+					Port:              1234,
+					PrivateInstanceId: "id-1",
+				})
+				pool.Put(noAlgoEndpoint)
+				Expect(pool.LoadBalancingAlgorithm).To(Equal(config.LOAD_BALANCE_HB))
+			})
 		})
 		It("returns the route_service_url associated with the pool", func() {
 			endpoint := &route.Endpoint{}