Core: Check referencedDataFile existence for DV

[ebyhr]

This field is required for DV. I missed setting this field when preparing deletion vector's changes in Trino, and it resulted in incorrect results even though DELETE statement succeeded. It would be nice to fail early.

iceberg/api/src/main/java/org/apache/iceberg/DeleteFile.java

Lines 35 to 44 in a04220a

	/**
	* Returns the location of a data file that all deletes reference.
	*
	* <p>The referenced data file is required for deletion vectors and may be optionally captured for
	* position delete files that apply to only one data file. This method always returns null for
	* equality delete files.
	*/
	default String referencedDataFile() {
	return null;
	}

[nastra]

+1, could you please add a small test for this?

[amogh-jahagirdar]

Thanks for finding and fixing this @ebyhr! +1 for adding a simple test

[ebyhr]

Thank you for your reviews! Added a test to TestDeleteFiles because there is no dedicated test class for FileMetadata & DeleteFile. Let me know if there is a better place or I should create a new test class.