FileMetaDataWidget: Avoid showing fields when they have no value

Summary:
When displaying remote:/ recentlyused:/, Search:/ or and Tags:/, the information panel shows modifications time, access time and size with empty field value.
This patches avoids displaying those fields when we have no value for them.

CCBUG: 413091
More precisely https://bugs.kde.org/show_bug.cgi?id=413091#c7

Test Plan:
Open remote:/ in Dolphin:
Size, Access and modification dates fields are not visible

Reviewers: Dolphin, ngraham, bruns

Reviewed By: Dolphin, ngraham, bruns

Subscribers: Baloo

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D28972