Is xmlns="" a valid xml namespace?

Is "empty" a valid value for XML namespace? If yes what does it mean?

I have the following XML code but I'm not sure to which namespace Field1 and Field2 elements belong to.

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
  <soapenv:Header />
  <soap:Body>
    <Root xmlns="uri">
      <Field1 xmlns="">147079737</Field1>
      <Field2 xmlns="">POL</Field2>
    </Root>
  </soap:Body>
</soapenv:Envelope>