KIO::iconNameForUrl: fix searching for kde protocol icons

Summary:
iconName was set to mt.iconName() at the beginning, but this returns always "application-octet-stream" making all subsequent iconName.isEmpty() check not work as expected.
This patch sets iconName to mt.iconName() only when needed.

BUG: 417922
BUG: 417921
CCBUG: 417069
FIXED-IN: 5.68

Test Plan:
In dolphin:

• open "Recent Files"
• Ctrl+T
• open any folder
• Ctrl+T
• Ctrl+F (Search
• Search for anything, type something then return

Same for smb:/ location

In all cases the protocol icon is now visible.

Reviewers: ngraham, Frameworks, dfaure, broulik, sitter

Reviewed By: ngraham, dfaure, sitter

Subscribers: sitter, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27539