mesh: update xds controller to synthesize empty endpoints when no endpoints ref is found #5810

Job	Run time
Get files changed and conditionally skip CI	32s
Setup	14s
Generate Envoy Job Matrices	7s
dev-build / build	1m 12s
nomad-integration-test (v1.6.1)	2m 9s
envoy-integration-test (1.27.0, server, case-wasm\|case-property-override\|case-cross-peers-http-ro...	5m 52s
nomad-integration-test (v1.5.8)	41s
envoy-integration-test (1.27.0, server, case-api-gateway-tcp-conflicted\|docs\|case-cfg-resolver-fe...	5m 20s
compatibility-integration-test	9m 2s
nomad-integration-test (v1.4.12)	38s
envoy-integration-test (1.27.0, server, case-terminating-gateway-simple\|case-cfg-resolver-cluster...	6m 35s
vault-integration-test (1.14.1)	2m 38s
envoy-integration-test (1.27.0, server, case-cfg-resolver-subset-redirect\|case-zipkin)	2m 13s
vault-integration-test (1.13.5)	2m 27s
envoy-integration-test (1.27.0, client, case-wasm\|case-property-override\|case-cross-peers-http-ro...	6m 2s
vault-integration-test (1.12.9)	2m 20s
envoy-integration-test (1.27.0, client, case-api-gateway-tcp-conflicted\|docs\|case-cfg-resolver-fe...	5m 22s
envoy-integration-test (1.27.0, client, case-terminating-gateway-simple\|case-cfg-resolver-cluster...	6m 25s
vault-integration-test (1.11.12)	2m 20s
envoy-integration-test (1.27.0, client, case-cfg-resolver-subset-redirect\|case-zipkin)	2m 7s
test-integrations-success	1s
	1h 4m 17s

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