Empty field visualisation in historical flow info details fixed

[YellowMan02]

Please sign (check) the below before submitting the Pull Request:

• [x] I have signed the ntop Contributor License Agreement at https://github.com/ntop/legal/blob/main/individual-contributor-licence-agreement.md
• [x] I have read the contributing guide lines https://github.com/ntop/ntopng/blob/dev/CONTRIBUTING.md
• [x] I have updated the documentation (in doc/src/) to reflect the changes made (if applicable)

Link to the related issue: https://github.com/ntop/ntopng/issues/8515

Describe changes: when a field is empty nothing is shown, a blank space was visualized before