How do I specify a nullable xs:int?

I have one xsd which defines element "Port" as "xs:int", and there are
bunch of xml files use "Port" element. But a lot of those files
contains <Port/>, which is treated invalid in xml validation.

I am wondering whether there is anyway to specify nullable integer, say
treat <Port/> the same as <Port>0</Port>?

Thanks in advance.

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