fix(core): fix nl2sql exception

[zhangshenghang]

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Caused by: java.lang.NullPointerException: null
	at java.base/java.util.Objects.requireNonNull(Objects.java:208) ~[na:na]
	at java.base/java.util.ImmutableCollections$MapN.<init>(ImmutableCollections.java:1186) ~[na:na]
	at java.base/java.util.Map.of(Map.java:1540) ~[na:na]
	at com.alibaba.cloud.ai.service.VectorStoreService.convertToDocument(VectorStoreService.java:286) ~[classes/:na]
	

Because the "samples" field was not set

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