Fix empty message spacing for attachment-only user messages #5580

Job	Run time
build-and-check	3m 3s
test-integration-packages/internal	1m 5s
test-integration-packages/billing	1m 8s
test-integration-cli	40s
test-integration-common	46s
test-packages/internal	44s
test-cli	1m 12s
test-packages/billing	35s
test-packages/agent-runtime	1m 30s
test-.agents	39s
test-integration-sdk	41s
test-integration-web	42s
test-web	49s
test-integration-packages/agent-runtime	1m 3s
test-common	38s
test-sdk	42s
	15m 57s

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