Resource should be optional in StaxEventItemReaderBuilder

[fmbenhassine]

Same as #3731 but for XML files.

[parikshitdutta]

Hi @benas, would you like to assign this to me? I can give it a try.

Thank you.

[parikshitdutta]

Hi @benas, Please have a look at my submitted PR #3752, I will look forward to your feedback.

Thank you.